feat: use utf8 charset for bundle response (#695)

Summary:
**Summary**

The content-type value of bundle server's response header should be 'application/javascript; charset=UTF-8', so that the browser can display characters normally when accessing bundle file.

**Test plan**

0. All unit tests pass.
1. Utf8 characters are displayed normally in the browsers.

Pull Request resolved: #695

Reviewed By: motiz88

Differential Revision: D30392590

Pulled By: feedthejim

fbshipit-source-id: 1caa7b5a0f78c8363db31daea98745d4f8083c3d

packages/metro/src/Server.js

@@ -790,7 +790,7 @@ class Server {
           String(result.numModifiedFiles),
         );
         mres.setHeader(DELTA_ID_HEADER, String(result.nextRevId));
-        mres.setHeader('Content-Type', 'application/javascript');
+        mres.setHeader('Content-Type', 'application/javascript; charset=UTF-8');
         mres.setHeader('Last-Modified', result.lastModifiedDate.toUTCString());
         mres.setHeader(
           'Content-Length',