Adds a keyword argument for explicitly setting the realm parameter for an OAuth Authorization header

My need for this specifically came up because the OAuth implementation for the QuickBooks Online API rejects a realm of "https://qbo.sbfinance.intuit.com" ('WWW-Authenticate: OAuth oauth_problem="parameter_rejected", oauth_parameters_rejected="realm"') as auto-constructed from a request to "https://qbo.sbfinance.intuit.com/resource/customer..." but accepts a realm of "qbo.sbfinance.intuit.com".

As this override option is implemented as a keyword argument with a default, the revised version of the library should be wholly backward-compatible.

Best,
Jon