Changed Upgrade from $ to [[, as $ is prefix based

[leibnitz27]

Chrome, since 44.0.2403.130, adds Upgrade-Insecure-Headers to basic http (NOT websocket) requests.

Previously, checks that a request was NOT a websocket upgrade were performed with

  # Web request
  if (is.null(J$wsinfo$Upgrade)) {
    # Not a handshake request, serve a static web page

Because R allows for prefix matching when performing $ lookups, this means the presence of Upgrade-Insecure-Headers fools R-Websockets into considering ALL requests to be websocket upgrades.

Use an exact match, via [["Upgrade"]] instead.