Update Fetch.js

#369

Resolution to the above issue, the RN code passes two instances of Content-Type, one containing capital letters and the other in lower case. 

The Android native code saves those options after applying .toLowerCase() on them.

The data in the lower case content-type overwrote the data in the Content-Type, and since the data on both the keys wasn't updated together at the fixed part of the code, it overwrote it with invalid data and caused the request to fail.

Author: nadav2051

polyfill/Fetch.js

@@ -40,6 +40,7 @@ class RNFetchBlobFetchPolyfill {
           promise = Blob.build(body).then((b) => {
             blobCache = b
             options.headers['Content-Type'] = 'multipart/form-data;boundary=' + b.multipartBoundary
+            options.headers['content-type'] = 'multipart/form-data;boundary=' + b.multipartBoundary
             return Promise.resolve(RNFetchBlob.wrap(b._ref))
           })
         }