set charset in the Content-Type header for chunked responses

[gregor-rayman]

Chunked plain text reponses would not include the charset part of the Content-Type header.

  val dummy: Endpoint[AsyncStream[String]] = get("dummy"){
    Ok( AsyncStream.of("Hello / Привет")).withCharset(StandardCharsets.UTF_8)
  }

The response of this endpoint would be correctly encoded in UTF-8, but the browsers would interpret it wrongly becuase of the missing charset part in the response header.

[codecov-io]

Codecov Report

Merging #938 into master will decrease coverage by 0.52%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #938      +/-   ##
==========================================
- Coverage   83.05%   82.52%   -0.53%     
==========================================
  Files          51       51              
  Lines         767      767              
  Branches       40       34       -6     
==========================================
- Hits          637      633       -4     
- Misses        130      134       +4

Impacted Files	Coverage Δ
core/src/main/scala/io/finch/Output.scala	94.73% <100%> (ø)	⬆️
core/src/main/scala/io/finch/Input.scala	87.5% <0%> (-12.5%)	⬇️
core/src/main/scala/io/finch/ToResponse.scala	96.77% <0%> (+3.22%)	⬆️

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[vkostyukov]

Looks great! Thank you, @gregor-rayman!