Handle more types for List.UniqueItems

To fix defects
  1. [MAGN-4640 List Unique Items for strings?] (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4640)
  2. [MAGN-5323 List.UniqueItems will not work on lists with null values](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5323)

For null value, we compare them separately with ``null``.

For other types that implements ``IConvertible``,  we call the corresponding convert to convert object to target type and do comparison. 

Related test cases added. 

@sharadkjaiswal  for you to review.