Headers are incorrectly serialized when @streaming=true

[JamesDunne]

When @streaming=true, header values are improperly serialized to HTTP as values like content-encoding: ["gzip"]

[krisrang]

Seeing this issue with passenger and images served through the rails webpacker gem. Webpack is sending a content-length header in the form of ['1271'].

[nertzy]

Is this issue resolved with the fix from #73? We should close this issue if so.