add allow_extra_fields option
feature #8133 [Form] Add allow_extra_fields option on forms (felds)

This PR was merged into the 2.5-dev branch.

Discussion
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[Form] Add allow_extra_fields option on forms

This is specially useful when creating different *nameless* GET forms on the same page (ex: pagination and search).

Commits
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c6f3dd6 add allow_extra_fields option
[Debug] generalized ErrorHandler
[Debug] update FrameworkBundle and HttpKernel for new ErrorHandler