Merge branch 'hotfix/#685-DBAL-997-undefined-_conn-property'

Close #685
Close #688
Close #691

lib/Doctrine/DBAL/Connection.php

@@ -597,7 +597,7 @@ class Connection implements DriverConnection
      */
     public function close()
     {
-        unset($this->_conn);
+        $this->_conn = null;
 
         $this->_isConnected = false;
     }

tests/Doctrine/Tests/DBAL/ConnectionTest.php

@@ -403,6 +403,13 @@ SQLSTATE[HY000]: General error: 1 near \"MUUHAAAAHAAAA\"");
         $this->assertSame($result, $conn->fetchColumn($statement, $params, $column, $types));
     }
 
+    public function testConnectionIsClosed()
+    {
+        $this->setExpectedException('Doctrine\\DBAL\\Exception\\DriverException');
+        $this->_conn->close();
+        $this->_conn->quoteIdentifier('Bug');
+    }
+
     public function testFetchAll()
     {
         $statement = 'SELECT * FROM foo WHERE bar = ?';