Move has_header_token() out of the WebSocket-only detail block and next to
the other header field helpers, forward-declaring it beside split(). Use the
existing split_find(), which drops the manual found flag and stops at the
first matching token.
Drive the client-side test from an ordinary Server route answering 101 with
a bad Connection value, rather than the hand-rolled listening socket copied
from the test above it.
d5c7d84cc4
