Support for Client Credentials Grant

[rgavinc]

Problem - unsupported feature

When making a call to obtain an access token using the "client_credentials" grant_type, the required information is not registered by Boxr::get_tokens

Boxr::get_tokens(grant_type: 'client_credentials', box_subject_type: 'enterprise', box_subject_id: '123455678')
*** ArgumentError Exception: unknown keywords: box_subject_type, box_subject_id

Solution

Allow keywords box_subject_type and box_subject_id which are required for Client Credentials

[xhocquet]

Hey @rgavinc , thanks for the report. If you're able to write a PR for this that matches our existing patterns, that would be great. Otherwise I'll keep an eye on this when I have some time to dedicate to Boxr!

[xhocquet]

Resolved with #120