[Form] Fixed DateType when used with the intl extension disabled

DateType's month select box returns timestamps when used with intl extension disabled (see #6485). 

The reason for this is that stubbed formats use *L* instead of *M* for month representation. My fix is simply taking this into account when building array for the select box.

I didn't provide unit tests since they're disabled when intl extension is not enabled. I've only manually verified if the months array contains correct data.

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: #6485
Todo: -
License of the code: MIT
Documentation PR: -