Don't intervene with non-AWS HTTP authentication

The HTTP Authorization header is a general purpose header, which also
has applications for http basic and digest authentications. Treating
all non-AWS authentication as a broken S3 request is the wrong thing
to do, as it is likely not even an S3 request. Instead, just pass it
on downstream.

Change-Id: I44d7f58604a3a4170bec13d98c2de10a6a034e56
Merge "Don't intervene with non-AWS HTTP authentication"