Don't include incoming querystring in Dropbox Business callback_url

[aguynamedben]

Unlike most providers, the Dropbox Business (and Dropbox consumer) API
require the callback_url to exactly match what is configured in their
web UI, including any querystring values. By default, OmniAuth appends any
incoming querystrings to the callback_url being sent the the
provider.

This means that if your app begins auths with something like:
/auth/dropbox_oauth2?auth_version=v2,

Your callback_url becomes:
/auth/dropbox_oauth2/callback?auth_version=v2

This doesn't exact match Dropbox Business' overly strict requirements
for this URL:
/auth/dropbox_oauth2/callback

The fix is for this provider to override callback_url so that the
querystring is not appended automatically.

There is a long-going disucssion to see whether this should be fixed
in omniauth-oauth2 or within each affected provider strategy:
omniauth/omniauth-oauth2#93

It's not super clear, but the consensus seems to be that this behavior
should be accounted for in the strategy.

Here's the similar issue for Dropbox (consumer):
icoretech/omniauth-dropbox2#2

Unmerged PR in the consumer library:
icoretech/omniauth-dropbox2#2

[nitanshu]

Thanks a lot @aguynamedben please check the master branch and let me know if it is working fine for you or not