Don't ignore previous headers when beginning OAuth

[paulomarg]

WHY are these changes introduced?

Fixes #651

As pointed out in the issue, there is currently no way of adding custom headers to a response when beginning OAuth on Node.js environments, because we overwrite all headers with the ones we need to set. We need to preserve any headers previously set by the app when adding our own.

WHAT is this pull request doing?

Changing the oauth begin process to start out from the incoming response via a new conversion method (defaulting to an empty one for non-node environments), and then adding our cookies / redirect headers on top of those rather ignoring all of them.

Type of change

• Patch: Bug (non-breaking change which fixes an issue)

Checklist

• I have added a changelog entry, prefixed by the type of change noted above
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)

[cquemin]

Nice fix!