Update HttpHeaders.getAccept method

Some servlet containers (iPlanet) parse the Accept header and return
multiple values from request.getHeader("Accept"). The HttpHeaders
getAccept method has been updated to accommodate that hopefully
without causing any other issues.

The extra functionality is in effect only if we find only one
MediaType and there is more than one value for the 'Accept' header.

Issue: SPR-9655
Fix exception message about producible media types

Issue: SPR-10175