Use 307 redirect when refreshing tokens in background to keep HTTP method

[swantzter]

Issue # (if available): fixes #82

Description of changes:

When redirecting to cognito, or we handle a request that's been redirected from cognito we likely want to try GET on the target since a) cognito expects it or b) if we've taken a detour to cognito the original method is lost anyway.
However, when we've refreshed tokens in the background and use a redirect response to the same url that was requested to set cookies we want the original request to be retried with the same method as it was originally made with, this way background POST/PUT/etc requests from web apps will not fail on the first try where tokens are refreshed

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