Honor Host header for server name/port in MckHtSrvRq

Prior to this commit, the getServerName() and getServerPort() methods
in MockHttpServletRequest simply returned the 'mocked' serverName and
serverPort but ignored the 'Host' header entirely. Per the Servlet
specification, however, these methods must parse the server name or
port from the 'Host' header if it is present and otherwise fall back to
the resolved server name or port.

This commit fixes this by ensuring that getServerName() and
getServerPort() properly parse the server's name or port from the
'Host' header if it is present in the request. If the 'Host' header is
not present, MockHttpServletRequest falls back to returning the
'mocked' serverName and serverPort.

Issue: SPR-12088