Fix dial panic when ctx is nil

Use net.ErrClosed

Go 1.16 has introduced net.ErrClosed, which should be returned/wrapped when an
I/O call is performed on a network connection which has already been closed.
This is useful to avoid cluttering logs with messages like "failed to close
WebSocket: already wrote close".

Closes: https://github.com/nhooyr/websocket/issues/286

Migrate from deprecated `io/ioutil`

Fixes https://github.com/nhooyr/websocket/issues/359

Credits to @maggie44 for making me add staticcheck.
See #407

Co-authored-by: maggie0002 <64841595+maggie0002@users.noreply.github.com>

Co-authored-by: lazypassion <25536767+lazypassion@users.noreply.github.com>

Cherry picked from master at 129d3035

Also ran gofmt on everything. Thanks again @paralin. #334

Co-authored-by: Christian Stewart <christian@paral.in>

Co-authored-by: Anmol Sethi <hi@nhooyr.io>

When the ctx is nil, http.NewRequestWithContext returns a "net/http:
nil Context" error and a nil request. In this case, the dial function
panics because it assumes the req is never nil. This checks the
returning error and returns it, so that callers get an error instead
of a panic in that scenario.

Co-authored-by: lazypassion <25536767+lazypassion@users.noreply.github.com>

Fix DOS attack from malicious pongs

A double channel close panic was possible if a peer sent back multiple
pongs for every ping.

If the second pong arrived before the ping goroutine deleted its channel
from the map, the channel would be closed twice and so a panic would
ensue.

This fixes that by having the read goroutine send on the ping
goroutine's channel rather than closing it.

Reported via email by Tibor Kálmán @kalmant

Please update to the new release ASAP!

Closes #247

Updates #255

netconn.go: Disable read limit on WebSocket