uniqueness with a field that has length less than "arbitrary_string" in code.

Hi all,
   I'm using shoulda and I was trying to test the uniqueness of an attribute that has a length { maximum: 11 }, but I was receiving the following error on rspec:

"cpf is too long (maximum is 11 characters) (\"arbitrary_string\")"]

So, I created a new method "use_string" to use my string instead of arbitrary string.

Sorry, this is my first pull request on github, if I forgot something, please tell me.
Thanks !!