[Image] Fix default Image functionality

Summary:
In the latest 0.9.0-rc of React Native, the default image won't load due to a typo and a missing condition in `setImage`. This PR contains fixes for both of them.
Closes https://github.com/facebook/react-native/pull/2269
Github Author: Tom Hastjarjanto <tom@intellicode.nl>

Libraries/Image/Image.ios.js

@@ -173,7 +173,7 @@ var Image = React.createClass({
         resizeMode={resizeMode}
         tintColor={tintColor}
         src={source.uri}
-        defaultSrc={defaultSource.uri}
+        defaultImageSrc={defaultSource.uri}
       />
     );
   }

Libraries/Image/RCTImageView.m

@@ -69,8 +69,9 @@ RCT_NOT_IMPLEMENTED(-init)
 
 - (void)setImage:(UIImage *)image
 {
+  image = image ?: _defaultImage;
   if (image != super.image) {
-    super.image = image ?: _defaultImage;
+    super.image = image;
     [self _updateImage];
   }
 }