`Changeset#update` should return `self`

[vladfaust]

Onyx::SQL.exec(user.update(changeset.update(foo: "bar")))

[vladfaust]

However, a developer could be confused with such a one-liner. Consider this example:

changeset = user.changeset
changeset.update(foo: "bar")
Onyx::SQL.exec(user.update(changeset.update(baz: "qux")))

It's not clear whether would the update take two fields or the baz only.