fix(button): prevent expression changed error on icon button

[evtkhvch]

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Short description of what this resolves:

Closes #2363

• fix NG0100 error in NbButton.iconRight

Error description. Error appears in the following case:

1. You use nbSpinner directive
2. <button> content contains <nb-icon> only
3. loading state changes

Example:

<button [nbSpinner]="loading" nbButton>
  <nb-icon icon="star"></nb-icon>
</button>

Problem: NbButton.iconRight has been changed after change detection has completed