Fix webob.exc.HTTPForbidden parameter miss

HTTPForbidden should use the parameter 'explanation'
instead of 'detail'.
This patch fixes this bug.

Change-Id: I0ebc01e8bbcc564debce450ea52faf94faa21eeb
Closes-Bug: #1283872
Removes a redundant test

I mistakenly asked for the test to be added in a review because I
thought there was some LDAP specific behavior. It turns out that this is
just a copy of the test in the parent class.

Change-Id: I98e007a1a40e64a18692623f80b7d733075c63c2
Don't need session.flush in context managed by session

Within the scope of a single method, keeping all the reads and writes
within
the context managed by a single session. In this way, the session's
__exit__
handler will take care of calling flush() and commit() for you.
If using this approach, you should not explicitly call flush() or
commit().
See http://docs.sqlalchemy.org/en/rel_0_7/orm/session.html#committing

Change-Id: I8e49179cb6cb71fc7b7c06799adcf9f10164a09b
Partial-Bug: #1258044
Merge "Don't need session.flush in context managed by session"
Merge "Removes a redundant test"
Merge "Fix webob.exc.HTTPForbidden parameter miss"