Pform

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- Use 'utf-8' as default form 'accept-charset'
- Support bootstrap3

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- Fixed composite and options templates for Chrome

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- Allow return additional data from action handler

- Pass different context to vocabulary factory
(depends on factory argument name: request, content, context)

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- Added `OptionsField`

- BaseMultiChoiceField.missing value is [] by default

- Copy field missing value during fieldset data extraction

- Fixed form csrf support

- Fixed 'checked' value for radio field

- Do not use missing value if value is not validation

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- Added `flat` attribute to field and fieldset,
with flat fieldset and composite field uses primary data
dictionary instead of sub dict

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- Fixed FileField validation