change android request builder so it wont set Content-Type header to …

[LorienHW]

…application/octet-stream if it was explicitly set to the empty string. This fixes amazon s3 uploads with a presigned url

This was fixed in a branch in the original project:
wkh237@3b58491

And submitted as an issue and tracked in wkh237#425

Its required for amazon s3 uploads using a secure upload url, which require the Content-Type header to be set to nothing. It will be called in the JS like so:

export function uploadFileToS3(presignedUrl, image) {
  const body = RNFetchBlob.wrap(image.uri.replace('file://', ''));
  return RNFetchBlob.fetch('PUT', presignedUrl,
    {
      'Content-Type': '',
    },
    body
  )
}

[Traviskn]

Thanks for your PR and explanation of the issue