fix(aws-lambda) strip headers that are disallowed by HTTP/2

We currently access Lambda over HTTP/1.1, and it returns a Connection
header as such. The Connection header is not used in HTTP/2 (it is
superseded by other protocol features) and including it violates RFC
7540 8.1.2.2.

Per the same "an intermediary transforming an HTTP/1.x
message to HTTP/2 will need to remove any header fields nominated by the
Connection header field, along with the Connection header field itself.",
we also remove additional headers. Lambda does not currently emit these,
and the RFC indicates it's not strictly necessary to strip them unless they're
nominated by Connection, but their presence will break some HTTP/2
implementations (libcurl and Chrome at least, possibly others), so stripping
them is future-proof against Lambda adding them in the future.

From #4032

kong/plugins/aws-lambda/handler.lua

@@ -129,12 +129,12 @@ function AWSLambdaHandler:access(conf)
   AWSLambdaHandler.super.access(self)
 
   local upstream_body = new_tab(0, 6)
+  local var = ngx.var
 
   if conf.forward_request_body or conf.forward_request_headers
     or conf.forward_request_method or conf.forward_request_uri
   then
     -- new behavior to forward request method, body, uri and their args
-    local var = ngx.var
 
     if conf.forward_request_method then
       upstream_body.request_method = var.request_method
@@ -230,6 +230,14 @@ function AWSLambdaHandler:access(conf)
   local content = res:read_body()
   local headers = res.headers
 
+  if var.http2 then
+    headers["Connection"] = nil
+    headers["Keep-Alive"] = nil
+    headers["Proxy-Connection"] = nil
+    headers["Upgrade"] = nil
+    headers["Transfer-Encoding"] = nil
+  end
+
   local ok, err = client:set_keepalive(conf.keepalive)
   if not ok then
     return responses.send_HTTP_INTERNAL_SERVER_ERROR(err)