Patch Adapter methods resolver

[dhilt]

This patch is a part of issue #213.
Running Adapter.clip (and other Adapter methods) before the Scroller is initialized did not resolve the returning Promise immediately, and the following code did fail:

this.datasource = new Datasource(...);
this.datasource.adapter.clip().then(() => {
  // never happened before this patch
  console.log('It needs to be resolved immediately');
});

This PR fixes it.