[GH-509] SchemaTool ignoring 'fixed' option

This issue is created automatically through a Github pull request on behalf of smottt:

Url: https://github.com/doctrine/doctrine2/pull/509

Message:

If the column is set to fixed string length like this:

```php
<?php

/**

• @ORM\Column(type="string", length=2, options= {"fixed" = true}

  )
  */
  protected $foo;
  ```

The schema tool will not detect that 'fixed' option correctly when generating update SQL.

Fix attached.