Fix Android 4.1-4.3 WebView source baseUrl bug

Resolves facebook#11753. Android versions 4.1-4.3 don't understand the MIME type 'text/html; charset=utf-8' and default to 'text/plain' instead, rendering the content as an unparsed HTML string. Since the encoding is already set and passed separately, removing it from the MIME type has no negative effects.

ReactAndroid/src/main/java/com/facebook/react/views/webview/ReactWebViewManager.java

@@ -87,7 +87,7 @@ public class ReactWebViewManager extends SimpleViewManager<WebView> {
   protected static final String REACT_CLASS = "RCTWebView";
 
   private static final String HTML_ENCODING = "UTF-8";
-  private static final String HTML_MIME_TYPE = "text/html; charset=utf-8";
+  private static final String HTML_MIME_TYPE = "text/html";
   private static final String BRIDGE_NAME = "__REACT_WEB_VIEW_BRIDGE";
 
   private static final String HTTP_METHOD_POST = "POST";
@@ -397,7 +397,7 @@ public void setMediaPlaybackRequiresUserAction(WebView view, boolean requires) {
   public void setAllowUniversalAccessFromFileURLs(WebView view, boolean allow) {
     view.getSettings().setAllowUniversalAccessFromFileURLs(allow);
   }
-  
+
   @ReactProp(name = "saveFormDataDisabled")
   public void setSaveFormDataDisabled(WebView view, boolean disable) {
     view.getSettings().setSaveFormData(!disable);
@@ -491,7 +491,7 @@ public void setMixedContentMode(WebView view, @Nullable String mixedContentMode)
         view.getSettings().setMixedContentMode(WebSettings.MIXED_CONTENT_ALWAYS_ALLOW);
       } else if ("compatibility".equals(mixedContentMode)) {
         view.getSettings().setMixedContentMode(WebSettings.MIXED_CONTENT_COMPATIBILITY_MODE);
-      } 
+      }
     }
   }