String support in THEN and ELSE part of CASE expression is broken

The SQL generated when using strings in the THEN and ELSE parts of the CASE expression are not valid.

This testcase will fail:
$this->assertSqlGeneration(
"SELECT g.id, CASE WHEN ((g.id / 2) > 18) THEN 'Foo' ELSE 'Bar' END AS test FROM Doctrine\Tests\Models\CMS\CmsGroup g",
"SELECT c0_.id AS id0, CASE WHEN (c0_.id / 2 > 18) THEN 'Foo' ELSE 'Bar' END AS sclr1 FROM cms_groups c0_"
);

Noticed by a user at github:
https://github.com/doctrine/doctrine2/pull/277
https://github.com/doctrine/doctrine2/pull/278