Improve failure message for assert_response_contains
Handle blank user email without blowing up.

Some of the validations were blowing up when email was nil.  There was
also an issue in devise_zxcvbn[1] relating to a nil email that was fixed in
version 1.1.1

[1] https://github.com/bitzesty/devise_zxcvbn/pull/5
Add some tests around changing email with blank
Merge pull request #218 from alphagov/handle_blank_emails

Handle blank user email without blowing up.