Changed behaviour of Str::addSuffix so it doesn't care about duplicate suffixes

[jzawadzki]

This PR tries to resolve the issue: https://github.com/symfony/maker-bundle/issues/964

I'm still not sure why Str::addSuffix wanted to be sure that suffix is not added twice. So, let someone with bigger knowledge of the bundle and class name generation review the PR :)

Thanks!

[OskarStark]

We should maybe have "addPrefix()" and "ensurePrefix()", where add adds it no matter the string looks before and ensure ensures it is only added once?

EnsureEnd() is the method used in Symfony string component