Don't map text/plain to csv. Refs #4105.

Jquery sets accepts header similar to "text/plain, */*; q=0.01" by
default for xhr requests. Due to this RequestHandler used to set
extension to csv thereby causing View class to look for views under
non-existent csv folders.

Cake/Network/Response.php

@@ -98,7 +98,7 @@ class Response {
 		'cpio' => 'application/x-cpio',
 		'cpt' => 'application/mac-compactpro',
 		'csh' => 'application/x-csh',
-		'csv' => array('text/csv', 'application/vnd.ms-excel', 'text/plain'),
+		'csv' => array('text/csv', 'application/vnd.ms-excel'),
 		'dcr' => 'application/x-director',
 		'dir' => 'application/x-director',
 		'dms' => 'application/octet-stream',
@@ -630,12 +630,12 @@ public function statusCode($code = null) {
 /**
  * Queries & sets valid HTTP response codes & messages.
  *
- * @param integer|array $code If $code is an integer, then the corresponding code/message is 
- *        returned if it exists, null if it does not exist. If $code is an array, then the 
- *        keys are used as codes and the values as messages to add to the default HTTP 
- *        codes. The codes must be integers greater than 99 and less than 1000. Keep in 
- *        mind that the HTTP specification outlines that status codes begin with a digit 
- *        between 1 and 5, which defines the class of response the client is to expect. 
+ * @param integer|array $code If $code is an integer, then the corresponding code/message is
+ *        returned if it exists, null if it does not exist. If $code is an array, then the
+ *        keys are used as codes and the values as messages to add to the default HTTP
+ *        codes. The codes must be integers greater than 99 and less than 1000. Keep in
+ *        mind that the HTTP specification outlines that status codes begin with a digit
+ *        between 1 and 5, which defines the class of response the client is to expect.
  *        Example:
  *
  *        httpCodes(404); // returns array(404 => 'Not Found')

Cake/Test/TestCase/Controller/Component/RequestHandlerComponentTest.php

@@ -145,6 +145,21 @@ public function testInitializeContentTypeWithjQueryAccept() {
 		$this->assertEquals('json', $this->RequestHandler->ext);
 	}
 
+/**
+ * Test that RequestHandler does not set extension to csv for text/plain mimetype
+ *
+ * @return void
+ */
+	public function testInitializeContentTypeWithjQueryTextPlainAccept() {
+		$_SERVER['HTTP_ACCEPT'] = 'text/plain, */*; q=0.01';
+		$event = new Event('Controller.initialize', $this->Controller);
+		$this->assertNull($this->RequestHandler->ext);
+		Router::parseExtensions('csv');
+
+		$this->RequestHandler->initialize($event);
+		$this->assertNull($this->RequestHandler->ext);
+	}
+
 /**
  * Test that RequestHandler sets $this->ext when jQuery sends its wonky-ish headers
  * and the application is configured to handle multiple extensions