[BlackBerry] Enable concatenating headers with same field name
https://bugs.webkit.org/show_bug.cgi?id=106625

Reviewed by Rob Buis.

RFC 2616 specifies that headers could concatenate with comma if they have
same field name. We should enable this if the header allows multiple values.

RIM PR# 275508, internally reviewed by Joe Mason

(WebCore):
(WebCore::isAppendableHeader):
(WebCore::NetworkJob::handleNotifyHeaderReceived):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139423 268f45cc-cd09-0410-ab3c-d52691b4dbfc