Add Test case for flexattr removal (#779)
Add defaut value to formatted mapping dict.

This mimics dict get method behavior.
Merge pull request #782 from PierreRust/fixRemoveFlexAttr

Fix remove flex attr
Revamp FormattedItemMapping

This subclass was not cleanly conforming to the Mapping abstract base. Now
we're in business -- the thing looks like a dict. Brought up by #782.