[docs] Add example usage for Route.HeadersRegexp (#320)

* Add example usage for Route.HeadersRegexp

* Improve example_route_test.go style

route.go

@@ -258,7 +258,8 @@ func (m headerRegexMatcher) Match(r *http.Request, match *RouteMatch) bool {
 //               "X-Requested-With", "XMLHttpRequest")
 //
 // The above route will only match if both the request header matches both regular expressions.
-// It the value is an empty string, it will match any value if the key is set.
+// If the value is an empty string, it will match any value if the key is set.
+// Use the start and end of string anchors (^ and $) to match an exact value.
 func (r *Route) HeadersRegexp(pairs ...string) *Route {
 	if r.err == nil {
 		var headers map[string]*regexp.Regexp