trivial fix
Convert birthday to DateField
Do not use 'SRegRequest' in views.

Use more abstract interface instead. This will allow us to implement
Attribute Exchange on top of it.
Support simple AX fetch request responses.
SReg/AX: ask only for available person data.

That is, don't ask for things we don't have a value for anyway.
Merge remote-tracking branch 'tampakrap/birthday' into openid-week5
SReg/AX: ignore empty/null values as well.