Add rfc7230 test suite and update others to recent Gun

This is a large commit.

The rfc7230 test suite adds many tests from the RFC7230 document.

Gun has been updated quite a bit recently, which broke the Cowboy
suites. This is now fixed with this commit.

A new hook onfirstrequest has been added. It was very useful during
debugging of the test suites.

The initial process code has changed a little; more changes are
expected with the switch to maps for options.

test/handlers/echo_h.erl

@@ -0,0 +1,16 @@
+%% This module echoes back the value the test is interested in.
+
+-module(echo_h).
+
+-export([init/2]).
+
+init(Req, Opts) ->
+	echo(cowboy_req:binding(key, Req), Req, Opts).
+
+echo(What, Req, Opts) ->
+	F = binary_to_atom(What, latin1),
+	Value = case cowboy_req:F(Req) of
+		V when is_integer(V) -> integer_to_binary(V);
+		V -> V
+	end,
+	{ok, cowboy_req:reply(200, [], Value, Req), Opts}.