unexpected behaviour on usage of .query on url with existing querystring

calling `.query(object)` with a url that has an existing query string will just append the query object to the existing url.

There is no documentation for this case so I assumed it would merge/override existing query parameters that came with the url.

I created this failing test case under test/request.js to demonstrate the issue:

```
it('GET querystring?search=Manny with query object with same query parameters', function(next){
request.get(uri + '/querystring?search=Manny')
.query({ search: 'Buzz' })
.end(function(err, res){
//assert.deepEqual(res.body, { search: 'Buzz'});
assert.equal(res.req.path, '/querystring?search=Buzz');
// ^^ the result is /querystring?search=Buzz&search=Manny
next();
});
});
```

**this happens since version 1.7.0.**

**behaviour on v1.6.1:**
original query string remains untouched, parameters from new query object that already exist in original are not appended.

what is your take on this?