Golang http server strips off 'Expect' header, if the
client sent this as part of signed headers we need to
handle otherwise we would see a signature mismatch.
`aws-cli` sets this as part of signed headers which is
a bad idea since servers trying to implement AWS
Signature version '4' will all encounter this issue.
According to
http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.20
Expect header is always of form:
Expect = "Expect" ":" 1#expectation
expectation = "100-continue" | expectation-extension
So it safe to assume that '100-continue' is what would
be sent, for the time being keep this work around.
8cdaf87c8f