add dedicated SQLServer2012PlatformTest and fix getSequenceNextValSQL in platform

lib/Doctrine/DBAL/Platforms/SQLServer2012Platform.php

@@ -75,7 +75,7 @@ class SQLServer2012Platform extends SQLServer2008Platform
      */
     public function getSequenceNextValSQL($sequenceName)
     {
-        throw 'SELECT NEXT VALUE FOR ' . $sequenceName;
+        return 'SELECT NEXT VALUE FOR ' . $sequenceName;
     }
 
     /**

tests/Doctrine/Tests/DBAL/Platforms/SQLServer2012PlatformTest.php

+<?php
+
+namespace Doctrine\Tests\DBAL\Platforms;
+
+use Doctrine\DBAL\Platforms\SQLServer2012Platform;
+use Doctrine\DBAL\Schema\Sequence;
+
+class SQLServer2012PlatformTest extends SQLServerPlatformTest
+{
+    public function createPlatform()
+    {
+        return new SQLServer2012Platform;
+    }
+
+    public function testSupportsSequences()
+    {
+        $this->assertTrue($this->_platform->supportsSequences());
+    }
+
+    public function testDoesNotPreferSequences()
+    {
+        $this->assertFalse($this->_platform->prefersSequences());
+    }
+
+    public function testGeneratesSequenceSqlCommands()
+    {
+        $sequence = new Sequence('myseq', 20, 1);
+        $this->assertEquals(
+            'CREATE SEQUENCE myseq START WITH 1 INCREMENT BY 20 MINVALUE 1',
+            $this->_platform->getCreateSequenceSQL($sequence)
+        );
+        $this->assertEquals(
+            'ALTER SEQUENCE myseq INCREMENT BY 20',
+            $this->_platform->getAlterSequenceSQL($sequence)
+        );
+        $this->assertEquals(
+            'DROP SEQUENCE myseq',
+            $this->_platform->getDropSequenceSQL('myseq')
+        );
+        $this->assertEquals(
+            "SELECT NEXT VALUE FOR myseq",
+            $this->_platform->getSequenceNextValSQL('myseq')
+        );
+    }
+}