Fix Kerberos Decryption Corruption

[Iristyle]

• The winrm_decrypt method, which appears to be a near copy of the original GSSAPI gem helpers from

  https://github.com/zenchild/gssapi/blob/master/examples

  includes a fatal bug.

  Specifically the original code at line https://github.com/zenchild/gssapi/blob/master/examples/gss_iov_helpers.rb#L50

  When breaking apart the given binary string response into length of header, header and payload - the incorrect value is given to Rubys unpack method.

  https://ruby-doc.org/core-2.3.0/String.html#method-i-unpack

  The directive 'A' is used:

  A | String | arbitrary binary string (remove trailing nulls and ASCII spaces)

  Since the given string is encrypted binary data, 'A' is the wrong directive given it performs removals.

  Instead, the directive 'a' should be used as it leaves all bytes intact:

  a | String | arbitrary binary string

• Without this change, intermittent failures will occur as the decrypted SOAP messages will contain almost valid XML, but usually end with corrupt binary strings at the end of otherwise valid UTF-8 like

  </s:Body></s\xB5f\xAF\x9B\xE5\x9B\xE9\xFE\xBB

  These failures occur frequently enough to make Kerberos usage completely unreliable

[Iristyle]

IMHO, this is one that could use a bugfix release @mwrock / @sneal. Thanks!

[Iristyle]

Docs for these APIs are at https://web.mit.edu/Kerberos/krb5-1.14/doc/appdev/gssapi.html#iov-message-wrapping

[mwrock]

Thanks a bunch Ethan! Will get a release out ASAP