Dirty hacky implementation of the type option.

Mime types need to be shared either via inheritance or by extracting theminto a separate class. This code is
horrible, but it works.

lib/Cake/Network/Http/Client.php

@@ -24,8 +24,27 @@
  *
  * ### Doing requests
  *
+ * ### Sending request bodies
+ *
+ * By default any POST/PUT/PATCH/DELETE request with $data will
+ * send their data as `multipart/form-data`.
+ *
+ * When sending request bodies you can use the `type` option to
+ * set the Content-Type for the request:
+ *
+ * `$http->get('/users', [], ['type' => 'json']);`
+ *
+ * The `type` option sets both the `Content-Type` and `Accept` header, to
+ * the same mime type. When using `type` you can use either a full mime
+ * type or an alias. If you need different types in the Accept and Content-Type
+ * headers you should set them manually and not use `type`
+ *
  * ### Using authentication
  *
+ *
+ * ### Using proxies
+ *
+ *
  */
 class Client {
 
@@ -268,6 +287,9 @@ protected function _createRequest($method, $url, $data, $options) {
 		$request->method($method)
 			->url($url)
 			->content($data);
+		if (isset($options['type'])) {
+			$request->header($this->_typeHeaders($options['type']));
+		}
 		if (isset($options['headers'])) {
 			$request->header($options['headers']);
 		}
@@ -278,4 +300,32 @@ protected function _createRequest($method, $url, $data, $options) {
 		return $request;
 	}
 
+/**
+ * Returns headers for Accept/Content-Type based on a short type
+ * or full mime-type.
+ *
+ * @param string $type short type alias or full mimetype.
+ * @return array Headers to set on the request.
+ */
+	protected function _typeHeaders($type) {
+		if (strpos($type, '/') !== false) {
+			return [
+				'Accept' => $type,
+				'Content-Type' => $type
+			];
+		}
+		// Hacky as hell but I'll clean it up I promise
+		$reflection = new \ReflectionClass('Cake\Network\Response');
+		$properties = $reflection->getDefaultProperties();
+		if (isset($properties['_mimeTypes'][$type])) {
+			$mimeTypes = $properties['_mimeTypes'][$type];
+			$mimeType = is_array($mimeTypes) ? current($mimeTypes) : $mimeTypes;
+			return [
+				'Accept' => $mimeType,
+				'Content-Type' => $mimeType,
+			];
+		}
+		return [];
+	}
+
 }

lib/Cake/Test/TestCase/Network/Http/ClientTest.php

@@ -277,4 +277,50 @@ public function testMethodsSimple($method) {
 		$this->assertSame($result, $response);
 	}
 
+/**
+ * Provider for testing the type option.
+ *
+ * @return array
+ */
+	public static function typeProvider() {
+		return [
+			['application/json', 'application/json'],
+			['json', 'application/json'],
+			['xml', 'application/xml'],
+			['application/xml', 'application/xml'],
+		];
+	}
+
+/**
+ * Test that using the 'type' option sets the correct headers
+ *
+ * @dataProvider typeProvider
+ * @return void
+ */
+	public function testPostWithTypeKey($type, $mime) {
+		$response = new Response();
+		$data = 'some data';
+		$headers = [
+			'Connection' => 'close',
+			'User-Agent' => 'CakePHP',
+			'Content-Type' => $mime,
+			'Accept' => $mime,
+		];
+
+		$mock = $this->getMock('Cake\Network\Http\Adapter\Stream', ['send']);
+		$mock->expects($this->once())
+			->method('send')
+			->with($this->logicalAnd(
+				$this->attributeEqualTo('_method', Request::METHOD_POST),
+				$this->attributeEqualTo('_content', $data),
+				$this->attributeEqualTo('_headers', $headers)
+			))
+			->will($this->returnValue($response));
+
+		$http = new Client([
+			'host' => 'cakephp.org',
+			'adapter' => $mock
+		]);
+		$http->post('/projects/add', $data, ['type' => $type]);
+	}
 }