case insensitive attribute mapping

[BitPatty]

Fixes an issue ( see #900 and #974 ) where attributes with uppercase characters would be filtered out in the response due to the lowercase matching in the search response:

https://github.com/ldapjs/node-ldapjs/blob/b86c493e7bbec6ae1d639951158645e1515467d7/lib/messages/search_response.js#L57-L71