http: move header serialization to http transport

http: move header serialization to http transport

Apps called http_serialize_headers, which creates plain text buffer,
this is now hidden in http transport layer and apps pass headers in
generic form, so they can be encoded based on http version.

Type: improvement

Change-Id: Ie4fa0516cd3406d60f956751c8ee7ab40e633fa4
Signed-off-by: Matus Fabian <[email protected]>