fix session.workstation when passing only domain

[mwrock]

This fixes:

C:/Users/MattWrock/AppData/Local/chefdk/gem/ruby/2.1.0/gems/rubyntlm-0.5.2/lib/net/ntlm/encode_util.rb:42:in `encode_utf16le': undefined method `encoding' for nil:NilClass (NoMethodError)

When passing in a :domain option

[mwrock]

pinging @zenchild or @sneal feel free to add me to this repo/rubygem and I can merge/release if you like.

[zenchild]

Looks good @mwrock. I also added you to the rubyntlm team so go ahead and merge.

[zenchild]

I pushed it up, but I also added you as an owner.

[mwrock]

thanks so much!