[docs] Clarify SetURLVars (#335)

* [docs] Clarify SetURLVars

Clarify in documentation that SetURLVars does not modify the given
*htttp.Request, provide an example of usage.

* Short and sweet function doc, example test.

test_helpers.go

@@ -7,7 +7,8 @@ package mux
 import "net/http"
 
 // SetURLVars sets the URL variables for the given request, to be accessed via
-// mux.Vars for testing route behaviour.
+// mux.Vars for testing route behaviour. Arguments are not modified, a shallow
+// copy is returned.
 //
 // This API should only be used for testing purposes; it provides a way to
 // inject variables into the request context. Alternatively, URL variables