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.
- mention that FOUND_ROWS is hardcoded; fixes #3146
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