Changing string to fixed string is not recognized in PostgreSQL Platform

The following change of an entity using PostgreSQL is not recognized by Doctrine DBAL:
from
``` php
    /**
     * @ORM\Column(type="string", name="name", length=2)
     */
    public $name;
```
to
``` php
    /**
     * @ORM\Column(type="string", name="name", length=2, options={"fixed"=true})
     */
    public $name;
```

A schema-update should change the column from `character varying(2)` to `character(2)` but instead acts like no changes were made on the column.

With this PR, I've extended the `PostgreSqlPlatform` to detect changes of the fixed option of a column.