fixed blobmanager inserting nil when there's no mime type
fixed blobmanager inserting nil when there's no mime type

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

hoastoolshop/react-native,facebook/react-native,javache/react-native,facebook/react-native,hoangpham95/react-native,hoangpham95/react-native,pandiaraj44/react-native,myntra/react-native,pandiaraj44/react-native,janicduplessis/react-native,facebook/react-native,javache/react-native,exponent/react-native,janicduplessis/react-native,pandiaraj44/react-native,pandiaraj44/react-native,hoastoolshop/react-native,myntra/react-native,myntra/react-native,hammerandchisel/react-native,exponent/react-native,hoangpham95/react-native,exponentjs/react-native,myntra/react-native,javache/react-native,hammerandchisel/react-native,hammerandchisel/react-native,hoastoolshop/react-native,facebook/react-native,arthuralee/react-native,exponentjs/react-native,facebook/react-native,hoangpham95/react-native,myntra/react-native,arthuralee/react-native,arthuralee/react-native,hoangpham95/react-native,facebook/react-native,pandiaraj44/react-native,exponentjs/react-native,janicduplessis/react-native,javache/react-native,myntra/react-native,exponentjs/react-native,javache/react-native,javache/react-native,exponentjs/react-native,pandiaraj44/react-native,exponent/react-native,myntra/react-native,hoastoolshop/react-native,janicduplessis/react-native,exponentjs/react-native,exponentjs/react-native,hoastoolshop/react-native,hoastoolshop/react-native,hammerandchisel/react-native,hoastoolshop/react-native,exponent/react-native,exponentjs/react-native,arthuralee/react-native,janicduplessis/react-native,javache/react-native,hoangpham95/react-native,hoangpham95/react-native,exponent/react-native,javache/react-native,hoangpham95/react-native,exponent/react-native,pandiaraj44/react-native,exponent/react-native,janicduplessis/react-native,javache/react-native,myntra/react-native,hammerandchisel/react-native,hammerandchisel/react-native,janicduplessis/react-native,hammerandchisel/react-native,myntra/react-native,hammerandchisel/react-native,arthuralee/react-native,facebook/react-native,facebook/react-native,pandiaraj44/react-native,hoastoolshop/react-native,exponent/react-native,janicduplessis/react-native,facebook/react-native