Question about Accept header and structured suffixes

RFC 6838 Section 4.2.8 defines Structured Syntax Name Suffixes, allowing
types to use a +
symbol to indicate they are subtypes of another media type such as
application/atom+xml and
application/xml. How do subtypes interact with the Accept header? Is a
server allowed to
respond to a request with Accept: application/xml with Content-Type:
application/atom+xml? I
imagine many clients compare the returned content type with their requested
content type with
a simple equality check, which would raise an error in this case. Is that
bad behavior on the
part of the client? If not, how should a client indicate that it accepts a
media type and any subtype
of that type?

P.S. This is my first post to the ietf-http-wg list. If this is not the
appropriate place for such a
question or there is some other issue with my post, please don't hesitate
to let me know.