Fix upload the file into blob field by using class Form

[ls1luke]

Fix to be able to actually upload a file into blob field by using class Form.

Given table definition as below

db.define_table('products',
    Field('name', 'string', length=50),
    Field('image', 'upload', uploadfield='image_data'),
    Field('image_data', 'blob', default=''))

and respective action

@action("new_product", method=["POST", "GET"])
@action.uses("new_product.html")
def new_product():
    form = Form(db.products)
    if form.accepted:
        redirect(URL('index'))
return dict(form=form)

when submitting the form, image_data column in database was always null.

The fix is just a copy of solution used in web2py.