equal not work properly when compare str with unicode

The following code snippet demonstrates the issue(run on OSX):

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

expect(abc['a']).to.equal('abc')
```

Run the above will generate the following exception message: 

```
BadExpectation: u'Expected "abc" to be "abc"'
```

If I replace equal to eq(namely, expect(abc['a']).to.eq('abc')), then it works
