add update() support to MutableDict
fix MutableDict.coerce

If a class inherited from MutableDict (say, for instance, to add an update() method), coerce() would give back an instance of MutableDict instead of an instance of the derived class.
Merge branch 'mutable-dict-coerce-fix' of https://bitbucket.org/goodscloud/sqlalchemy into pr27
- changelog for pr bitbucket:27
Merge branch 'mutable-dict-update' of https://bitbucket.org/goodscloud/sqlalchemy into pr27
- changelog for pr bitbucket:28