Replace type of method parameter of HttpClient::SendRequest with enum

This way we advertise subset of of methods we are going to use.

BUG:24267885
Change-Id: I4dda539269b490d0f6a828342ee9182f539b225b
Reviewed-on: https://weave-review.googlesource.com/1294
Reviewed-by: Vitaly Buka <vitalybuka@google.com>