Checkman stops polling upon getting a 504

[CardboardMechanic]

Checkman seems to stop polling when using the site script and the response is a 504 (I believe this happens with a 502 too, but didn't have logs to show it). This results in a red build until Checkman is restarted.

Example log output from Alt+clicking the build in question:

cd /Users/pivotal/Dropbox/front-end; /bin/bash "-lc" "PATH=$PATH:/Applications/Checkman.app/Contents/Resources site.check https://console.run.pivotal.io/heartbeat"

{"result":false,"changing":false,"url":"https://example.com/heartbeat","info":[["Url","https://example.com/heartbeat"],["Status","HTTP/1.1 504 Gateway Timeout"],["Content-length:0"]]}

stty: stdin isn't a terminal
stty: stdin isn't a terminal
curl -sIk -A "CheckmanSite (Hostname: $HOSTNAME)" 'https://example.com/heartbeat'
HTTP/1.1 504 Gateway Timeout

Content-length:0

[CardboardMechanic]

This could also be a [lack of?] header problem. In the case of 504/502 Checkman doesn't actually get to the app. In the case where Checkman reaches to our app and gets a 500, it behaves as expected.