Merge pull request #24 from 4teamwork/lg-set-default-values-before-adding-content-to-container

DxBuilder: Make sure default values are set before adding content to container
DxBuilder: Fix check if field is present (to determine if default values should be set).

(field.get() *will* raise an AttributeError if the field doesn't exist on obj, and
can't be provided with a default value)