Mark all translations as translated when the target locale matches the source language

Hi, 
this PR solve https://github.com/schmittjoh/JMSTranslationBundle/issues/197 issue.

When the source language matches the target locale, then all translations should be already considered as translated.

If you are using some kind of placeholders instead of message strings (eg: `label.button.text`) then, your source language should be `null`...