code: func (r *Route) Headers(pairs ...string) *Route {
  	if r.err == nil {
  		var headers map[string]string
  		headers, r.err = mapFromPairsToString(pairs...)
  		return r.addMatcher(headerMatcher(headers))
  	}
  	return r
  }
docs: // Headers adds a matcher for request header values.
  // It accepts a sequence of key/value pairs to be matched. For example:
  //
  //     r := mux.NewRouter()
  //     r.Headers("Content-Type", "application/json",
  //               "X-Requested-With", "XMLHttpRequest")
  //
  // The above route will only match if both request header values match.
  // If the value is an empty string, it will match any value if the key is set.