Allow custom parser (#49)

Author: f3ath

lib/src/client.dart

@@ -16,14 +16,15 @@ typedef http.Client HttpClientFactory();
 class JsonApiClient {
   static const contentType = 'application/vnd.api+json';
 
-  final JsonApiParser _parser = const JsonApiParser();
+  final JsonApiParser _parser;
 
   final HttpClientFactory _factory;
 
   /// JSON:API client uses Dart's native Http Client internally.
-  /// To customize its behavior you can pass the [factory] function.
-  const JsonApiClient({HttpClientFactory factory})
-      : _factory = factory ?? _defaultFactory;
+  /// To customize its behavior you can pass the [factory] function and the [parser].
+  const JsonApiClient({HttpClientFactory factory, JsonApiParser parser})
+      : _factory = factory ?? _defaultFactory,
+        _parser = parser ?? const JsonApiParser();
 
   /// Fetches a resource collection by sending a GET request to the [uri].
   /// Use [headers] to pass extra HTTP headers.