[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.

mux_test.go

@@ -2248,6 +2248,15 @@ func TestMethodsSubrouterPathVariable(t *testing.T) {
 	}
 }
 
+func ExampleSetURLVars() {
+	req, _ := http.NewRequest("GET", "/foo", nil)
+	req = SetURLVars(req, map[string]string{"foo": "bar"})
+
+	fmt.Println(Vars(req)["foo"])
+
+	// Output: bar
+}
+
 // testMethodsSubrouter runs an individual methodsSubrouterTest.
 func testMethodsSubrouter(t *testing.T, test methodsSubrouterTest) {
 	// Execute request

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