Default keepalive to infinity for all protocols

For HTTP/2 some servers are not fond of receiving pings,
especially if there's no stream at the time. (particularly
gRPC servers). This is an odd decision for sure. Rather
than trying to accomodate for their logic the ping is no
longer sent by default.

For Websocket sending a ping is a new feature in Gun 2.0
so I'm just being conservative there.

doc/src/manual/gun.asciidoc

@@ -241,7 +241,7 @@ flow - see below::
 The initial flow control value for all HTTP/2 streams.
 By default flow control is disabled.
 
-keepalive (5000)::
+keepalive (infinity)::
 
 Time between pings in milliseconds.
 
@@ -514,7 +514,7 @@ flow - see below::
 The initial flow control value for the Websocket connection.
 By default flow control is disabled.
 
-keepalive (5000)::
+keepalive (infinity)::
 
 Time between pings in milliseconds.
 
@@ -559,7 +559,7 @@ when receiving a ping.
          lowercased as well.
 * *2.0*: Function `gun:headers/4,5` introduced.
 * *2.0*: The `keepalive` option is now set to `infinity` by
-         default for the HTTP/1.1 protocol, disabling it.
+         default for all protocols. This means it is disabled.
 * *2.0*: Websocket options `keepalive` and `silence_pings` introduced.
 * *1.3*: Add the CONNECT destination's `protocols` option and
          deprecate the previously introduced `protocol` option.

src/gun_http2.erl

@@ -164,7 +164,7 @@ do_check_options([Opt={Name, _}|Opts]) ->
 name() -> http2.
 opts_name() -> http2_opts.
 has_keepalive() -> true.
-default_keepalive() -> 5000.
+default_keepalive() -> infinity.
 
 init(_ReplyTo, Socket, Transport, Opts0) ->
 	%% We have different defaults than the protocol in order

src/gun_ws.erl

@@ -91,7 +91,7 @@ do_check_options([Opt|_]) ->
 name() -> ws.
 opts_name() -> ws_opts.
 has_keepalive() -> true.
-default_keepalive() -> 5000.
+default_keepalive() -> infinity.
 
 init(ReplyTo, Socket, Transport, #{stream_ref := StreamRef, headers := Headers,
 		extensions := Extensions, flow := InitialFlow, handler := Handler, opts := Opts}) ->