@joel.verhagen

"Should HTTPResponse.getheaders() comma-separate the values (...)"

No, it should not. RFC 2616 states:

"Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]."

As field-values for some header fields ('Set-Cookie' being an example) are not defined as a comma-separated list such fields must not be merged.

Side note:
RFC 2616 is very soon to be obsoleted by the new RFC from httpbin working group. However, in the current/newest draft (http://trac.tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#section-3.2) although wording is different the sense is the same.