Code review comment for lp:~mabac/launchpad/login-raises-non-string

In checking the method the only possible options are either an email address or the lp.testing.ANONYMOUS which is a string as well.

I asked in IRC the driving idea behind the change and your explanation of clearing up a potential confusing error message seems potentially helpful in debugging the failing test driven from passing a non-email string to the method.

As noted in irc by salgado, basestring would be a much better comparison since often this will be a unicode string and not a strict str. Please update that.