datastore: make query objects mutable

[callmehiphop]

Related to #806

This makes Query objects mutable.

[stephenplusplus]

Do any of the tests test for this being returned?

[stephenplusplus]

👏

[callmehiphop]

Do any of the tests test for this being returned?

They don't explicitly test that what is returned is the same as the original object e.g.

var query = new Query(['kind1']);
var limited = query.limit(10);

assert.strictEqual(query, limited);

However, there are tests that make use of the chaining - https://github.com/GoogleCloudPlatform/gcloud-node/blob/master/test/datastore/query.js#L345

[stephenplusplus]

It's better to test for it explicitly, in case something random goes wrong with that catch all test. (maybe the last one in the chain doesn't return this, maybe not all of the methods were used, etc)

[callmehiphop]

Sure, tests have been created!