Fix handling of four byte utf8 manifests

- Adds the charset: utf-8 to all the manifest responses
- Makes sure we connect to MySQL in utf8mb4 mode, to ensure we can properly read and write 4-byte utf8 strings
- Adds tests for all of the above

endpoints/v2/manifest.py

@@ -76,7 +76,7 @@ def fetch_manifest_by_tagname(namespace_name, repo_name, manifest_ref):
     supported.bytes.as_unicode(),
     status=200,
     headers={
-      'Content-Type': supported.media_type,
+      'Content-Type': '%s; charset=utf-8' % supported.media_type,
       'Docker-Content-Digest': supported.digest,
     },
   )
@@ -111,7 +111,7 @@ def fetch_manifest_by_digest(namespace_name, repo_name, manifest_ref):
   metric_queue.repository_pull.Inc(labelvalues=[namespace_name, repo_name, 'v2', True])
 
   return Response(supported.bytes.as_unicode(), status=200, headers={
-    'Content-Type': supported.media_type,
+    'Content-Type': '%s; charset=utf-8' % supported.media_type,
     'Docker-Content-Digest': supported.digest,
   })