Mimic CURLOPT_POST in GuzzleHttpRequest

Mimic CURLOPT_POST in GuzzleHttpRequest

The MWHttpRequest is implemented by the
CurlHttpRequest class and also the
GuzzleHttpRequest class. However, curl based rendering set
the CURLOPT_POST which implies that the 'Content-Type'
header defaults to 'application/x-www-form-urlencoded'.
To homgonize the functionality this patch mimics the
curl behaviour in Guzzle.

Bug: T232866
Change-Id: Id60a8de18e5f1e750a3bde23bd8b0deca4071165
(cherry picked from commit 5e3a0e73955d6324c5dd6e12fbe36d3ba203d9db)