Encode client and domain in oem/unicode in Client#authenticate!

[jlee-r7]

I'm using my SMB2 implementation to exercise this code. Type1 messages have to have the domain/workstation encoded as OEM (ascii), so we need to pass them in as either "binary" or "utf-8" encoding to the Client constructor. However, when we build the Type3 message, it has to be in whatever encoding was negotiated, usually Unicode ("utf-16le").

Without this change, passing correct user/pass:

• domain encoded as ascii-8bit or utf-8 results in STATUS_LOGON_FAILURE
• domain encoded as unicode results in this stack trace:

Encoding::CompatibilityError: incompatible character encodings: ASCII-8BIT and UTF-16LE
    from /home/egypt/repo/rubyntlm/lib/net/ntlm/message.rb:90:in `serialize'
    from /home/egypt/repo/smb2/lib/smb2/client.rb:72:in `authenticate'
    from (irb):2
    from /home/egypt/.rvm/rubies/ruby-2.1.5/bin/irb:11:in `<main>'

After this change, passing domain as ascii-8bit or utf-8 works as expected.

[zenchild]

@jlee-r7 Thanks for the fix and the testing.