[Android] Removed duplicate code in RNFetchBlobBody.java

Removed duplicated code at line 107

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 .