Default header configuration.

class HttpDefaultHeaders {
 static String _defaultContentType = 'application/json;charset=utf-8';
 Map _headers = {
   'COMMON': {
       'Accept': 'application/json, text/plain, */*'
   },
   'POST' : {
       'Content-Type': _defaultContentType
   },
   'PUT' : {
     'Content-Type': _defaultContentType
   },
   'PATCH' : {
     'Content-Type': _defaultContentType
   }
 };

 _applyHeaders(method, ucHeaders, headers) {
   if (!_headers.containsKey(method)) return;
   _headers[method].forEach((k, v) {
     if (!ucHeaders.contains(k.toUpperCase())) {
       headers[k] = v;
     }
   });
 }

 /**
  * Called from [Http], this method sets default headers on [headers]
  */
 setHeaders(Map<String, String> headers, String method) {
   assert(headers != null);
   var ucHeaders = headers.keys.map((x) => x.toUpperCase()).toSet();
   _applyHeaders('COMMON', ucHeaders, headers);
   _applyHeaders(method.toUpperCase(), ucHeaders, headers);
 }

 /**
  * Returns the default header [Map] for a method.  You can then modify
  * the map.
  *
  * Passing 'common' as [method] will return a Map that contains headers
  * common to all operations.
  */
 operator[](method) {
   return _headers[method.toUpperCase()];
 }
}