array-types field-name generation

If you have an entity form field which has "multiple" set to true, the bundle fails to generate an array-type field name (meaning: we'll get "myEntity" instead of "myEntity[]").

However, i could need some help to improve the PR:
I found it pretty hard to test the entity type, so the only thing that worked for me was extending the actual doctrine entity-type. Therefore, we now have a composer-dep to doctrine in require-dev, just for the test to work. 
There might be a better way to solve this.

Also, this change affects the expected results in the FormTypeParserTest; Seems legit to me, but could need a review ^^
This makes it impossible to test against older version of Symfony