change android request builder so it wont set Content-Type header to application/octet-stream if it was explicitly set to the empty string. This fixes amazon s3 uploads with a presigned url

android/src/main/java/com/RNFetchBlob/RNFetchBlobReq.java

                     requestType = RequestType.Form;
                 }
                 else if(cType.isEmpty()) {
-                    builder.header("Content-Type", "application/octet-stream");
+                    if(!cType.equalsIgnoreCase("")) {
+                      builder.header("Content-Type", "application/octet-stream");
+                    }
                     requestType = RequestType.SingleFile;
                 }
                 if(rawRequestBody != null) {