HTTP codec auto may incorrectly select http/2

[zuercher]

When the HTTP connection manager codec is set to type AUTO, it searches the first buffer on the connection for the string PRI * HTTP/2. If that string occurs anywhere in the buffer, Envoy attempts to negotiate HTTP/2 and fails. If the string appears in an HTTP header or request body it can trigger HTTP/2 incorrectly.

Repro steps:

Start envoy with the config below and issue a request containing the string:

$ curl -X POST --data "something something PRI * HTTP/2" localhost:8000/foo
curl: (52) Empty reply from server

Log reports:

[2019-09-12 14:59:50.891][3032142][debug][http] [external/envoy/source/common/http/conn_manager_impl.cc:284] [C4] dispatch error: Received bad client magic byte string

Config:

node:
  id: "plain"
  cluster: "plain"
static_resources:
  listeners:
    - name: plain
      address:
        socket_address:
          protocol: TCP
          address: 0.0.0.0
          port_value: 8000
      filter_chains:
        - filters:
            - name: envoy.http_connection_manager
              config:
                idle_timeout: 1s
                codec_type: auto
                stat_prefix: ingress_http
                route_config:
                  name: plain
                  virtual_hosts:
                    - name: plain
                      domains:
                        - "*"
                      routes:
                        - match:
                            prefix: "/"
                          route:
                            cluster: plain
                http_filters:
                  - name: envoy.router
                    typed_config: {}

  clusters:
    - name: plain
      connect_timeout: 0.25s
      type: STATIC
      lb_policy: round_robin
      hosts:
        - socket_address:
            address: "127.0.0.1"
            port_value: 8080
admin:
  access_log_path: "/dev/null"
  address:
    socket_address:
      address: 0.0.0.0
      port_value: 9000

[zuercher]

Basically, we need to modify this: https://github.com/envoyproxy/envoy/blob/master/source/common/http/conn_manager_utility.cc#L34