Removed Buffer call with 'binary' encoding option in auth example.

According to the Node.js documentation for Buffer objects regarding the
'binary' encoding option:

"This encoding method is deprecated and should be avoided in favor of
Buffer objects where possible. This encoding will be removed in future
versions of Node."

Simply calling toString() with a 'base64' argument on the hash seems to
accomplish the same thing; this makes the code compatible with current
documentation as well as being a bit easier to follow.
Merge pull request #1822 from yakubori/auth-buffer-call-removal

Removed Buffer call with 'binary' encoding option in auth example.