pkg/shelf: change helper method on Request and Response

pkg/shelf/lib/src/request.dart

Index: pkg/shelf/lib/src/request.dart
diff --git a/pkg/shelf/lib/src/request.dart b/pkg/shelf/lib/src/request.dart
index f7e011dc42609321c775d6d8ecc9bc7af582e78a..57f255e975fdc87b906bab5c19dfcb56a32493e1 100644
--- a/pkg/shelf/lib/src/request.dart
+++ b/pkg/shelf/lib/src/request.dart
@@ -81,8 +81,7 @@ class Request extends Message {
             headers: headers, context: context) {
     if (method.isEmpty) throw new ArgumentError('method cannot be empty.');
 
-    // TODO(kevmoo) use isAbsolute property on Uri once Issue 18053 is fixed
-    if (requestedUri.scheme.isEmpty) {
+    if (!requestedUri.isAbsolute) {
       throw new ArgumentError('requstedUri must be an absolute URI.');
     }
 
@@ -107,6 +106,27 @@ class Request extends Message {
       throw new ArgumentError('scriptName and url cannot both be empty.');
     }
   }
+
+  /// Creates a new [Request] by copying existing values and applying specified
+  /// changes.
+  ///
+  /// New key-value pairs in [context] and [headers] will be added to the copied
+  /// [Request].
+  ///
+  /// If [context] or [headers] includes a key that already exists, the
+  /// key-value pair will replace the corresponding entry in the copied
+  /// [Request].
+  ///
+  /// All other context and header values from the [Request] will be included
+  /// in the copied [Request] unchanged.
+  Request change({Map<String, String> headers, Map<String, Object> context}) {
+    headers = updateMap(this.headers, headers);
+    context = updateMap(this.context, context);
+
+    return new Request(this.method, this.requestedUri,
+        protocolVersion: this.protocolVersion, headers: headers, url: this.url,
+        scriptName: this.scriptName, body: this.read(), context: context);
+  }
 }
 
 /// Computes `url` from the provided [Request] constructor arguments.