Add support for omitting scope

[vakhtang]

Hey all, it'd be nice if there existed an ability to omit the scope, since scope is optional. Based on the spec it looks like the scope cannot be empty if a value is being passed, so introducing Scope.empty or a similar value is probably not the right thing to do here, although that'd be the most expedient solution.

[blondacz]

This is to omit scope from a request but same needs to be done on response, as the scope on response is required only when different from scope in request

[majk-p]

Implemented in #199, released as v0.14.0-RC1