Changes

library/HTTP_Adapter: do not hard-code 'Content-Type'

* Do not send `Content-Type` when request contains no body.
** This is wrong and some servers would reject such a request.
* Allow passing custom `Content-Type` to
** `f_http_tx_request()` and
** `f_http_transact()`.
* Use `application/json` by default.

Change-Id: Ie35ffc56b4fedc0b4d8c4a689a2232b515b7b326