Public test API to set URL params (#322)

* Add a function to set url params for test

* [docs] add justification for SetURLVars and description of alternative approach to setting url vars.
* rename SetURLParams to SetURLVars as this is more descriptive.
* rename testing to testing_helpers as this is more descriptive.

* [docs] add stipulation to SetURLVars that it should only be used for testing purposes

test_helpers.go

@@ -0,0 +1,18 @@
+// Copyright 2012 The Gorilla Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mux
+
+import "net/http"
+
+// SetURLVars sets the URL variables for the given request, to be accessed via
+// mux.Vars for testing route behaviour.
+//
+// This API should only be used for testing purposes; it provides a way to
+// inject variables into the request context. Alternatively, URL variables
+// can be set by making a route that captures the required variables,
+// starting a server and sending the request to that server.
+func SetURLVars(r *http.Request, val map[string]string) *http.Request {
+	return setVars(r, val)
+}