do not transform HTTP POST requests to GETs for proxy'd paths

[fool]

Customer noticed that their function, which differentiates POST handling from GET handling, when used on a site with prerendering configured, translates POST to GET which of course broke the function.

I think we MUST handle POST-as-GET for facebook's tendency to use POST to fetch content and that being most of the point of prerendering, but I think we need not transform it for other services, as a customer demonstrated in https://netlify.zendesk.com/agent/tickets/16082 which has a good repro setup.

[fool]

from the customer:

• I set up 2 new Netlify sites, one with prerender on and one without and attached the same repo to them both.
• I set the webhook to call: https://webhook-prerender-debug.netlify.com/.netlify/functions/shipstation-get-rates
• Observed the POST from the webhook logged as a GET with 'user-agent': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/74.0.3729.108 Safari/537.36 Prerender (+https://github.com/prerender/prerender)'
• Turned prerender off and observed the same behavior, which was unexpected. Turned prerender back on.
• Set the webhook to call the NO prerender site's function at: https://webhook-debug-no-prerender.netlify.com/.netlify/functions/shipstation-get-rates
• Observed the POST cam across as expected with 'user-agent': 'Snipcart/1.0'

Deploy for each:
https://app.netlify.com/sites/webhook-prerender-debug/deploys/5e17847713be790008724477
https://app.netlify.com/sites/webhook-debug-no-prerender/deploys/5e1784fb84f0c10188a514cc

[fool]

adding this to the traffic & delivery platform board; if that turns out to be the wrong place, please at mention me and remove it from there so I can try to find a better place (I am not sure if it is really a bug in the prerender service where I filed it - I think it might be a proxy bug since we proxy to prerender? Advice sought :))

If you remove it from that board because it isn't yours, and don't say something about doing so, it will get lost and that would be sad.