Fix assertUniqueName throwing exception when using MultiWidget

[Spawin]

Here is a fix for the problem encountered here 61 (I encountered the same problem). I went through all the related discussions and considered all aspects (like here what assertUniqueName is supposed to be doing? ):

• Fix for MultiWidget
• Respect conditions for checkboxes and radios
• Run all tests

Note: I will make another PR for processing data from a MultiWidget itself.