Ported source prop over to iOS WebView

Summary:
public
https://github.com/facebook/react-native/pull/5494 added a new `source` property to WebView on Android that provides a better API, as well as allowing for request headers to be set.

This diff ports that functionality over to iOS, so we can have a consistent API cross-platform.

I've also extended the API to include `method` (GET or POST) and `body` when setting the WebView content with a URI, and `baseUrl` when setting static HTML.

Reviewed By: javache

Differential Revision: D2884643

fb-gh-sync-id: 83f24494bdbb4e1408aa8f3b7428fee33888ae3a

React/Base/RCTConvert.m

@@ -118,8 +118,31 @@ RCT_CUSTOM_CONVERTER(NSData *, NSData, [json dataUsingEncoding:NSUTF8StringEncod
 
 + (NSURLRequest *)NSURLRequest:(id)json
 {
-  NSURL *URL = [self NSURL:json];
-  return URL ? [NSURLRequest requestWithURL:URL] : nil;
+  if ([json isKindOfClass:[NSString class]]) {
+    NSURL *URL = [self NSURL:json];
+    return URL ? [NSURLRequest requestWithURL:URL] : nil;
+  }
+  if ([json isKindOfClass:[NSDictionary class]]) {
+    NSURL *URL = [self NSURL:json[@"uri"] ?: json[@"url"]];
+    if (!URL) {
+      return nil;
+    }
+    NSData *body = [self NSData:json[@"body"]];
+    NSString *method = [self NSString:json[@"method"]].uppercaseString ?: @"GET";
+    NSDictionary *headers = [self NSDictionary:json[@"headers"]];
+    if ([method isEqualToString:@"GET"] && headers == nil && body == nil) {
+      return [NSURLRequest requestWithURL:URL];
+    }
+    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
+    request.HTTPBody = body;
+    request.HTTPMethod = method;
+    request.allHTTPHeaderFields = headers;
+    return [request copy];
+  }
+  if (json) {
+    RCTLogConvertError(json, @"a valid URLRequest");
+  }
+  return nil;
 }
 
 + (RCTFileURL *)RCTFileURL:(id)json