Update the query when updating the selection.

query gets updated when selection changes as a computed property.
However, when a user changes the query and selects the same selection,
query does not get updated. This change fixes that issue.

@Dylnuge 
please let me know if there is a better way to do this other than changing selection to empty string and back.
Why do we need this line if immediately below it we override it with `@get('content')`?