args change for httpclient_req:new/4, fix and add tests

httpclient_req:new/4 didn't fall in line with its /3 and /5
equivalents. /3 accepts Method, Headers, Path args, while /4
accepts Method, Headers, Path, Params, Body. It makes sense for /3
to accept Method, Headers, Path, Params and not Method, Headers,
Path, Body.