Set the length for MyType implementation

Mysql drops the type in these tests, when it does visit_typeclause,
 since it's an unkown type it just says none,
and doesn't do a cast.
Firebird also doesn't support varchar with length, it throws an
error on these types.

test/sql/test_types.py

@@ -558,7 +558,7 @@ class TypeCoerceCastTest(fixtures.TablesTest):
     @classmethod
     def define_tables(cls, metadata):
         class MyType(types.TypeDecorator):
-            impl = String
+            impl = String(50)
 
             def process_bind_param(self, value, dialect):
                 return "BIND_IN" + str(value)