fields.Constant is confusing

[fayazkhan]

fields.Contsant only works for fields which are already present in the object.

from marshmallow import fields, Schema, pprint

class Test(Schema):
    field = fields.Constant(0)
    field2 = fields.Constant(1)

pprint(Test().load({'field': 1}))

I'd expect this code to give me:

UnmarshalResult(data={'field': 0, 'field2': 1}, errors={})

But this is what I get:

UnmarshalResult(data={'field': 0}, errors={})

If this is the expected behavior, it isn't really clear from the docs.
So either the docs or the behavior ought to be changed.

[sloria]

This is indeed a bug. Fixed in a69622c. Thanks @fayazkhan for reporting.