Summary:
Upon handling symbolication response when a redbox occurs, the blob manager inserted `nil` mime type because the response header doesn't specify any. This crashed the app with exception:
```
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[4]'
terminating with uncaught exception of type NSException
```
Reviewed By: shergin
Differential Revision: D6945632
fbshipit-source-id: 9298bd2674447106763c73e1a5035417bd30f29c
854c2330eb