WebRequest::getHeader: add optional flag to get back list

WebRequest::getHeader: add optional flag to get back list

* Add a stub FauxRequest::initHeaders method. This allows FauxRequest to
  use WebRequest's getHeader implemention, leading to less code duplication.
* Add an optional $flags parameter to WebRequest::getHeader. When the flag
  WebRequest::GETHEADER_LIST is set, treat the header as a comma-separated
  list of values (per RFC 2616 § 4.2) and return an array.
  (Initial use-case: Ibb292d241's treatment of If-None-Match header.)
* Add test.

See <http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html>.

Change-Id: I2760f57adfa7b886c84044a6e9348a47ac307229