add unit test for http2 connection
add unit test for http2 connection

libcurl handles http2 connections transparently. So this just puts a
unit test in place to make sure it doesn't break with configuration
changes or anything like that.

fixes #49

mrtazz/restclient-cpp,mrtazz/restclient-cpp,embeddedmz/restclient-cpp,embeddedmz/restclient-cpp