Ensure let block is not passed to super call.

Fix #820. This is directly related to bug
http://bugs.ruby-lang.org/issues/8059.  It seems to affect Ruby 1.9.3
but not Ruby 2.0.0. As per the notes, it is a heisenbug, so simply
looking at it causes the bug to disappear.

Force the call to super to make sure the let block is not passed in.
Update Changelog with issue description.
Merge pull request #827 from cupakromer/fix-issue-820

Ensure let block is not passed to super call.