Handle decompression when using Req.Test + :plug + compress_body: true

[davoclavo]

Hello!

After updating a project to v0.5.15 I noticed that things break if Req.Test is used with a plug and we set compress_body: true. This seems to be happening because a Plug.Parsers.call that was recently added to automatically parse the body; so in order to fix that we just need to make sure it is decompressed before doing so.

This is the error that shows up in the test I added (before the fix was applied)

  1) test compress_body compress_body works with plug adapter (Req.StepsTest)
     test/req/steps_test.exs:395
     ** (Plug.Parsers.ParseError) malformed request, a Jason.DecodeError exception was raised with message "unexpected byte at position 0: 0x1F"
     code: resp = Req.post!(plug: plug, json: %{test: "data"}, compress_body: true)
     stacktrace:
       (plug 1.16.1) lib/plug/parsers/json.ex:95: Plug.Parsers.JSON.decode/3
       (plug 1.16.1) lib/plug/parsers.ex:340: Plug.Parsers.reduce/8
       (req 0.5.15) lib/req/steps.ex:1042: Req.Steps.run_plug/1
       (req 0.5.15) lib/req/request.ex:1118: Req.Request.run_request/1
       (req 0.5.15) lib/req/request.ex:1062: Req.Request.run/1
       (req 0.5.15) lib/req.ex:1119: Req.request!/2
       test/req/steps_test.exs:403: (test)

[wojtekmach]

Thank you so much, apologies for delay in getting this in!