Fix duplicate content-type in server response

[guymguym]

Signed-off-by: Guy Margalit [email protected]

Motivation and Context

• Content-Type header was set twice in the server response to S3 GetObject/HeadObject causing the client to fail.
• This happens for operations that bind an output member to the content-type header.
• Even though the call to set_response_header_if_absent checks for dup header, it was called before the output members, and the output members code did not check for dups, and added the header twice.

Description

• By replacing the order so that the default content-type is added after setting the output members and only if absent, the duplicate is detected on runtime and avoided.

Testing

• Tested with S3 GetObject

Checklist

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[david-perez]

@guymguym Thank you.

I've made a note in #1164 (comment) to ensure we don't forget to add a protocol test to not regress on this, once we have the "extras" suite hooked up.