reference https://github.com/square/okhttp/issues/2738

Lost content-length header fix

When install in RN > 0.30 the link script will automatically add a
statement to Android HTTP response callback (in RNFetchBlobReq.java) so
that response body does not leaking.

Use a more sophisticated implementation as @timsuchanek mentioned in
#90

Prevent app crash when form data contains undefined data field #90 .

- Fix Blob constructor when mixed array contains undefined element
- Fix onCreate event invoke issue when Blob is created by Blob.slice

Related to #89 , when the request body is a Blob instance, we should
check if the Blob is created before the request actually send.

For test #89  use a larger test image.