added RegExp comparison so you can provide RegExp in post request spec

I really need this feature that allows you to give a request spec as a RegExp (actually, only by string)
So you can write cool stuff like:
```javascript
var scope = nock('http://www.google.com')
    .post('/echo', 'key=v.?l')
    .reply(200, function(uri, body) {
        return 'OK !';
    });
```
And be able to match requests like `POST http://www.google.com/path key=val`or `key=vul`, or even `key=vl`if you want. 
If the provided pattern is actually not a valid RegExp, then nock considers it as a basic string.