Fix for Unicode decoding issue when using incremental networking.

Summary:
This is **a critical issue**.

The issue arises when incremental networking is enabled from JS by setting `onprogress` or `onload` on an `XMLHttpRequest` object.

The results:

![example1](https://cloud.githubusercontent.com/assets/2270433/18829964/5a54ff30-83e7-11e6-9806-97857dce0430.png)

![example2](https://cloud.githubusercontent.com/assets/2270433/18829966/5bf40a66-83e7-11e6-84e6-9e4d76ba4f8b.png)

Unicode characters get corrupted seemingly in random. The issue is from the way Unicode character parsing is handled in `RCTNetworking.mm`. When incremental networking is enabled, each chunk of data is decoded and passed to JS:

```objective-c
incrementalDataBlock = ^(NSData *data, int64_t progress, int64_t total) {
NSString *responseString = [RCTNetworking decodeTextData:data fromResponse:task.response];
if (!responseString) {
  RCTLogWarn(@"Received data was not a string, or was not a recognised encoding.");
  return;
}
NSArray<id> *responseJSON = @[task.requestID, responseString, @(prog
Closes https://github.com/facebook/react-native/pull/10110

Reviewed By: yungsters

Differential Revision: D4101533

Pulled By: fkgozali

fbshipit-source-id: 2674eaf0dd4568889070c6cde5cdf12edc5be521

Examples/UIExplorer/UIExplorerUnitTests/RCTUnicodeDecodeTests.m

@@ -0,0 +1,77 @@
+/**
+ * The examples provided by Facebook are for non-commercial testing and
+ * evaluation purposes only.
+ *
+ * Facebook reserves all rights not expressly granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NON INFRINGEMENT. IN NO EVENT SHALL
+ * FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#import <XCTest/XCTest.h>
+
+#import "RCTNetworking.h"
+
+static NSString* const niqqudStringB64 = @"15HWsNa816jWtdeQ16nWtNeB15nXqiwg15HWuNa816jWuNeQINeQ1rHXnNa515TWtNeZ150sINeQ1rXXqiDXlNa316nWuNa814HXnta315nWtNedLCDXldaw15DWtdeqINeU1rjXkNa416jWttelLg==";
+
+@interface RCTNetworking ()
+
++ (NSString *)decodeTextData:(NSData *)data fromResponse:(NSURLResponse *)response withCarryData:(NSMutableData*)inputCarryData;
+
+@end
+
+@interface RCTUnicodeDecodeTests : XCTestCase
+
+@end
+
+@implementation RCTUnicodeDecodeTests
+
+- (void)runTestForString:(NSString*)unicodeString usingEncoding:(NSString*)encodingName cutAt:(NSUInteger)cutPoint
+{
+  CFStringEncoding cfEncoding = CFStringConvertIANACharSetNameToEncoding((CFStringRef)encodingName);
+  NSStringEncoding encoding = CFStringConvertEncodingToNSStringEncoding(cfEncoding);
+  
+  NSData* unicodeBytes = [unicodeString dataUsingEncoding:encoding];
+  
+  NSURLResponse* fakeResponse = [[NSHTTPURLResponse alloc] initWithURL:[NSURL URLWithString:@"testurl://"]
+                                                            statusCode:200
+                                                           HTTPVersion:@"1.1"
+                                                          headerFields:@{@"content-type": [NSString stringWithFormat:@"text/plain; charset=%@", encodingName]}];
+  XCTAssert([fakeResponse.textEncodingName isEqualToString:encodingName]);
+  
+  NSMutableData* carryStorage = [NSMutableData new];
+  NSMutableString* parsedString = [NSMutableString new];
+  
+  [parsedString appendString:[RCTNetworking decodeTextData:[unicodeBytes subdataWithRange:NSMakeRange(0, cutPoint)]
+                                              fromResponse:fakeResponse
+                                             withCarryData:carryStorage] ?: @""];
+  
+  [parsedString appendString:[RCTNetworking decodeTextData:[unicodeBytes subdataWithRange:NSMakeRange(cutPoint, unicodeBytes.length - cutPoint)]
+                                              fromResponse:fakeResponse
+                                             withCarryData:carryStorage] ?: @""];
+  
+  XCTAssert(carryStorage.length == 0);
+  XCTAssert([parsedString isEqualToString:unicodeString]);
+}
+
+- (void)testNiqqud
+{
+  NSString* unicodeString = [[NSString alloc] initWithData:[[NSData alloc] initWithBase64EncodedString:niqqudStringB64
+                                                                                               options:(NSDataBase64DecodingOptions)0]
+                                                  encoding:NSUTF8StringEncoding];
+  
+  [self runTestForString:unicodeString usingEncoding:@"utf-8" cutAt:25];
+}
+
+- (void)testEmojis
+{
+  NSString* unicodeString = @"\U0001F602\U0001F602";
+  
+  [self runTestForString:unicodeString usingEncoding:@"utf-8" cutAt:7];
+}
+
+@end