[NNTP] added IHAVE support
[NNTP] added more documentation for NNTP methods
[NNTP] made NNTP#close private
[request_spec] fixed response class for BODY
[NNTP] added support for AUTHINFO USER/PASS
[NNTPClient] added ::start and #connect
[NNTPClient] fixed references to NNTPRequest subclasses

This broke when class was renamed to NNTPClient