permessage-deflate support

[Mixfair]

Hi.
Sharing my example of implementing permessage-deflate using CodecZlib in continuation #853.
In the process, I encountered fragmentation difficulties, since the opcode is calculated during the process of sending data and we do not know about the type of compressed data. Therefore, I decided to separately compress a string into a string, an array into an array before sending. Also moved utf-8 check after unpacking. Tested with Chrome and Julia client.

Please, review it

Server:

WebSockets.listen(host, port; binary=false, isdeflate=true) do ws
    @async while true
        sleep(2)
        payload = WebSockets.compress("Hello")                       # if fragmentation true
        WebSockets.send(ws, [payload[1:2], payload[3:end]])
        # payload = "Hello"                                          # if fragmentation false
        # WebSockets.send(ws, payload)
    end
    for msg in ws
        @info "Received: ", msg
    end
end

Client:

WebSockets.open("ws://127.0.0.1:8080") do ws
    for msg in ws
        @info "Client received: ", msg
        # msg = WebSockets.compress(msg)                            # if fragmentation true
        WebSockets.send(ws, msg)
    end
end

[codecov-commenter]

Codecov Report

Merging #868 (fb7111e) into master (cf706bc) will decrease coverage by 3.88%.
The diff coverage is 60.81%.

❗ Current head fb7111e differs from pull request most recent head c3a3b7c. Consider uploading reports for the commit c3a3b7c to get more accurate results

@@            Coverage Diff             @@
##           master     #868      +/-   ##
==========================================
- Coverage   80.12%   76.23%   -3.89%     
==========================================
  Files          36       36              
  Lines        2878     2925      +47     
==========================================
- Hits         2306     2230      -76     
- Misses        572      695     +123     

Impacted Files	Coverage Δ
src/WebSockets.jl	51.82% <31.70%> (-36.00%)	⬇️
src/clientlayers/ConnectionRequest.jl	78.94% <90.00%> (+2.32%)	⬆️
src/ConnectionPool.jl	88.44% <100.00%> (-0.51%)	⬇️
src/HTTP.jl	74.32% <100.00%> (-8.73%)	⬇️
src/Handlers.jl	84.14% <100.00%> (+0.29%)	⬆️
src/clientlayers/RetryRequest.jl	74.41% <0.00%> (-2.33%)	⬇️
src/Messages.jl	87.03% <0.00%> (-0.62%)	⬇️
src/cookiejar.jl	77.01% <0.00%> (-0.58%)	⬇️
... and 1 more

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