Fix to allow query parameters to idp login url to be maintained

[mjsalinger]

Fixes #46 by adding any query parameters in the passed-in login url to the resulting generated url.

[cozmo]

Unless I'm missing something I think this logic could be cleaned up a bit by replacing lines 501-512 with the following. Thoughts?

uri = url.parse identity_provider.sso_login_url, true
if options.sign_get_request
  _(uri.query).extend sign_request(deflated.toString('base64'), @private_key, options.relay_state)
else
  uri.query.SAMLRequest = deflated.toString('base64')
  uri.query.RelayState = options.relay_state if options.relay_state?
uri.search = null # If you provide search and query search overrides query :/

It's possible I'm missing some intricacy of the logic here though and if so please correct me :D

[cozmo]

Closing this as I've fixed it in #79 - Thanks for the initial PR. Published as 1.8.1 on npm.