Introduce BufferedResponse filter

examples/app/app.vala

@@ -30,6 +30,14 @@ app.get ("", (req, res, next) => {
 	template.to_stream (res.body);
 });
 
+app.get ("buffered", (req, res, next) => {
+	res.headers.set_content_length (resources_lookup_data ("/templates/home.html", ResourceLookupFlags.NONE).length);
+	next (req, new VSGI.BufferedResponse (res));
+}).then((req, res) => {
+	var template = new View.from_resources ("/templates/home.html");
+	template.to_stream (res.body);
+});
+
 app.get ("gzip", (req, res, next) => {
 	res.headers.replace ("Content-Encoding", "gzip");
 	next (req, new VSGI.ConvertedResponse (res, new ZlibCompressor (ZlibCompressorFormat.GZIP)));

src/vsgi-buffered-response.vala

@@ -0,0 +1,50 @@
+using GLib;
+using Soup;
+
+namespace VSGI {
+
+	/**
+	 * Buffers the body of a response.
+
+	 * Unlike the traditional {@link VSGI.Response}, the head is not written
+	 * when the body is accessed, but instead flushed. Modifications to the
+	 * status line and headers can still occur even if content has been written
+	 * to the body stream.
+
+	 * The default size of the buffer is set to the value of the 'Content-Length'
+	 * header or defaults to 4 kilobytes if it hasn't been set.
+	 *
+	 * If the 'Content-Length' header is set in the response
+	 * The default size of the buffer is
+	 *
+	 * @since 0.3
+	 */
+	public class BufferedResponse : FilteredResponse {
+
+		public BufferedOutputStream buffer { construct; get; }
+
+		public override OutputStream body {
+			get {
+				return this.buffer;
+			}
+		}
+
+		/**
+		 *
+		 *
+		 * @since 0.3
+		 */
+		public BufferedResponse (Response base_response) {
+			base_response.head_written = true; // trick the response into
+			var buffer = base_response.headers.get_encoding () == Encoding.CONTENT_LENGTH ?
+				new BufferedOutputStream.sized (base_response.body, (size_t) base_response.headers.get_content_length ()) :
+				new BufferedOutputStream (base_response.body);
+			Object (base_response: base_response, buffer: buffer);
+		}
+
+		public BufferedResponse.sized (Response base_response, uint buffer_size) {
+			base_response.head_written = true; // trick the response into
+			Object (base_response: base_response, buffer: new BufferedOutputStream.sized (base_response.body, buffer_size));
+		}
+	}
+}

src/vsgi-response.vala

@@ -53,7 +53,7 @@ namespace VSGI {
 		 *
 		 * @since 0.2
 		 */
-		public bool head_written { get; protected set; default = false; }
+		public bool head_written { get; protected construct set; default = false; }
 
 		/**
 		 * Response body.