unicode string should be treated as string

I encouter a problem with the following code snippet(run in OSX):

``` python
from robber import expect
import json
abc=json.loads('{"a": "a string"}')

expect(abc['a']).to.be.a.string()
```

the above expect statement will generate an exception:
`
BadExpectation: u'Expected "a string" to be a string'
`

which does not act as expected in my opinion.
