Escape URL string before creating an NSURL

Summary:
If we make a request that contains a pipe character, URLWithString will return null.  This fix escapes these and other characters before creating the NSURL.
Closes https://github.com/facebook/react-native/pull/786
Github Author: Blair Vanderhoof <blairv@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

React/Base/RCTConvert.m

@@ -58,18 +58,14 @@ RCT_CONVERTER(NSString *, NSString, description)
 
 + (NSURL *)NSURL:(id)json
 {
-  if (![json isKindOfClass:[NSString class]]) {
-    RCTLogError(@"Expected NSString for NSURL, received %@: %@", [json classForCoder], json);
-    return nil;
-  }
-
-  NSString *path = json;
+  NSString *path = [self NSString:json];
   if ([path isAbsolutePath])
   {
     return [NSURL fileURLWithPath:path];
   }
   else if ([path length])
   {
+    path = [path stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
     NSURL *URL = [NSURL URLWithString:path relativeToURL:[[NSBundle mainBundle] resourceURL]];
     if ([URL isFileURL] && ![[NSFileManager defaultManager] fileExistsAtPath:[URL path]]) {
       RCTLogWarn(@"The file '%@' does not exist", URL);