Remove un-necessary Set::merge().

Using Set::merge() on an empty array causes issues with out of order
numeric keys. Only merge if necessary.

Fixes #2595

lib/Cake/Network/CakeRequest.php

@@ -145,7 +145,14 @@ public function __construct($url = null, $parseEnvironment = true) {
 
 /**
  * process the post data and set what is there into the object.
- * processed data is available at $this->data
+ * processed data is available at `$this->data`
+ *
+ * Will merge POST vars prefixed with `data`, and ones without 
+ * into a single array. Variables prefixed with `data` will overwrite those without.
+ *
+ * If you have mixed POST values be careful not to make any top level keys numeric
+ * containing arrays. Set::merge() is used to merge data, and it has possibly
+ * unexpected behavior in this situation.
  *
  * @return void
  */
@@ -165,8 +172,14 @@ protected function _processPost() {
 			}
 			unset($this->data['_method']);
 		}
+		$data = $this->data;
 		if (isset($this->data['data'])) {
 			$data = $this->data['data'];
+		}
+		if (count($this->data) <= 1) {
+			$this->data = $data;
+		}
+		if (count($this->data) > 1) {
 			unset($this->data['data']);
 			$this->data = Set::merge($this->data, $data);
 		}

lib/Cake/Test/Case/Network/CakeRequestTest.php

@@ -177,6 +177,19 @@ public function testPostParsing() {
 		$_POST = array('one' => 1, 'two' => 'three');
 		$request = new CakeRequest('some/path');
 		$this->assertEquals($_POST, $request->data);
+
+		$_POST = array(
+			'data' => array(
+				'Article' => array('title' => 'Testing'),
+			),
+			'action' => 'update'
+		);
+		$request = new CakeRequest('some/path');
+		$expected = array(
+			'Article' => array('title' => 'Testing'),
+			'action' => 'update'
+		);
+		$this->assertEquals($expected, $request->data);
 	}
 
 /**