After clearSearch nextSearchTerm is not cleared

https://github.com/ivaynberg/select2/issues/2753
Fixed not to break use case: setting the new value after someone was just searching with a different value

https://github.com/ivaynberg/select2/issues/2751
Dont update the results if the term hasn't changed.

https://github.com/ivaynberg/select2/issues/2751
Merge pull request #2755 from kupishkis/2753

After clearSearch nextSearchTerm is not cleared