Add coerce argument to SelectField in model_form

[ddorian]

For example if you have a intfield with choices, it shows an error

[rozza]

Pull requests welcome! Or can you clarify..

[ddorian]

I looked at this and couldn't find an automatic way to fix it. To reproduce is simple:

1. Create document with IntField(with_choices)
2. Create model_form from document.
3. Submit the form.

You need to make:

class DOC(db.Document):
     CHOICES = ((1,'string'),(2,'another string'))
     thefield = IntField(choices=CHOICES)

DocumentForm = model_form(DOC, field_args={
    'thefield': {'coerce': int}
})

Basically the resulting SelectField() must have (coerce=int) but i don't know how to make it automatic for booleans,floats etc. (or it doesn't have to be automatic?)

[rozza]

Added in 0.7