Fix sortKey() not looking at default params.

sortKey() only looked at the options values, which do not include
default sorting options declared as settings in the helper.

Fixes #2640

lib/Cake/Test/Case/View/Helper/PaginatorHelperTest.php

@@ -53,6 +53,8 @@ public function setUp() {
 					'prevPage' => false,
 					'nextPage' => true,
 					'pageCount' => 7,
+					'order' => null,
+					'limit' => 20,
 					'options' => array(
 						'page' => 1,
 						'conditions' => array()
@@ -369,6 +371,21 @@ public function testSortKey() {
 		$this->assertEquals($result, 'Article');
 	}
 
+/**
+ * Test that sortKey falls back to the default sorting options set
+ * in the $params which are the default pagination options.
+ *
+ * @return void
+ */
+	public function testSortKeyFallbackToParams() {
+		$this->Paginator->request->params['paging']['Article']['order'] = 'Article.body';
+		$result = $this->Paginator->sortKey();
+		$this->assertEquals('Article.body', $result);
+
+		$result = $this->Paginator->sortKey('Article');
+		$this->assertEquals('Article.body', $result);
+	}
+
 /**
  * testSortDir method
  *

lib/Cake/View/Helper/PaginatorHelper.php

@@ -199,13 +199,14 @@ public function sortKey($model = null, $options = array()) {
 			$params = $this->params($model);
 			$options = $params['options'];
 		}
-
 		if (isset($options['sort']) && !empty($options['sort'])) {
 			return $options['sort'];
-		} elseif (isset($options['order']) && is_array($options['order'])) {
-			return key($options['order']);
-		} elseif (isset($options['order']) && is_string($options['order'])) {
-			return $options['order'];
+		}
+		if (isset($options['order'])) {
+			return is_array($options['order']) ? key($options['order']) : $options['order'];
+		}
+		if (isset($params['order'])) {
+			return is_array($params['order']) ? key($params['order']) : $params['order'];
 		}
 		return null;
 	}