ws: Added the validation of HTTP Host header

Added the check for presence of mandatory 'Host' header as expected by HTTP 1.1.
Together with a test case "test_webserver_host_header" in test-webserver.c

Fixes #663
Signed-off-by: Stef Walter <stefw@redhat.com>
 * Fixed trailing whitespace
 * Put cocpit_assert_expected() at top of teardown()