Make the selectivity setter helper to clear value before setting a new one on single value mode on ftest framework

When we go to a document and try to edit a single value field the behaviour is not the same as if the field was empty.

How to reproduce

• go to webui
• create a document and in the nature field start to write Assessment
• Assessment will appear in the dropdown in blue
• Press enter key, Assessment is selected, save.
• go and edit the document again
• go to the nature field and start to write Card
• Card will appear in the dropdown but not highlighted in blue
• press enter key, Card is not selected.
• You need to select it with the mouse or down arrow key to save and replace the nature

see video for more info