[devtools] make return value of get_response_header(raw=True) consistent wrt get_header(raw=True) i.e. get_response_header(raw=True) now returns the first element of the list rather than the list itself.

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile