Supporting :create_scopes with :prefix option

Hi Cassio!

If I have reserved word in my Enumeration (e.g. new). When I try to use both options, the `create_scopes` method tries to create scope with the reserved word and exception is launched.

Tks!
Is the refactoring related to the code changed in this PR? If so, you can make the changes now. If it's not, then go for it :-)