[Doctrine] fixed ValueConversion strategy handing

Strategy 0 for DQL will throw an exception now as its not supported (see Github #26)
Merge pull request #27 from rollerworks/doctrine-conversion-strategy-fix

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no (strategy pattern did not work before this)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT