When using git-over-http the GitHttpClientController would try to look
up the user or token read from the Authorization headers.

If one of those headers would contain a base64 encoded null-byte,
this would result in an ArgumentError.

This adds support for that to the middleware by decoding the
authorization headers and validating them beforehand.

It will also avoid trying to decode non-base64 encoded headers, and
instead validate the content without as-is.

This reverts commit 44cebe45.