Prevent mutable fields from incorrectly becoming dirty.

Fixes a bug in Studio by allowing us to reliably tell whether or not a
field has been explicitly set. Previously, because accessing a mutable
field marked it dirty, it was impossible to tell if a field was
actually modified by a course author.
Reword comment.
Merge pull request #57 from edx/peter-fogg/model-data-bug

Prevent mutable fields from incorrectly becoming dirty.