Remove Content Type from Http Body

[dxuhuang]

@DanSimon @benblack86 @aliyakamercan @amotamed @jlbelmonte

Second stab at https://github.com/tumblr/colossus/issues/545

So what we'd like to be able to do is set the content-type on the HttpResponse without having to bother the internal HttpBody. I don't see an inherent reason why the content-type has to be part of HttpBody anyway; after all, the only substantial place it's used is in the encode() method that writes to the DynamicOutBuffer, and the body itself is just a byte array. If we add the content-type Header properly, HttpResponseHead.encode() will take care of it just the same.

The only issue I see is potential duplicate headers since HttpHeaders is just a wrapper around a linked list, but I think that should still be handled in HttpHeaders alone. In fact I'm thinking about going a bit further and making HttpHeaders wrap a HashMap[String, HttpHeader] instead.

Discuss.

[codecov-io]

Codecov Report

Merging #569 into develop will increase coverage by 0.22%. The diff coverage is 83.33%.

@@             Coverage Diff             @@
##           develop     #569      +/-   ##
===========================================
+ Coverage    84.69%   84.91%   +0.22%     
===========================================
  Files           98       99       +1     
  Lines         4422     4427       +5     
  Branches       360      348      -12     
===========================================
+ Hits          3745     3759      +14     
+ Misses         677      668       -9

Impacted Files	Coverage Δ
...rc/main/scala/colossus/service/ServiceClient.scala	91.6% <ø> (-2.8%)	:arrow_down:
...n/scala/colossus/controller/OutputController.scala	88.09% <0%> (ø)	:arrow_up:
...rc/main/scala/colossus/protocols/http/Header.scala	84.02% <0%> (+0.69%)	:arrow_up:
...a/colossus/protocols/http/HttpResponseParser.scala	68.18% <100%> (+0.32%)	:arrow_up:
.../main/scala/colossus/protocols/http/HttpBody.scala	85.71% <100%> (+6.54%)	:arrow_up:
...la/colossus/protocols/http/HttpRequestParser.scala	95.23% <100%> (+3.23%)	:arrow_up:
...n/scala/colossus/protocols/http/HttpResponse.scala	89.13% <100%> (+0.24%)	:arrow_up:
...s/src/main/scala/colossus/core/server/Server.scala	95.07% <0%> (-0.71%)	:arrow_down:
...src/main/scala/colossus/metrics/StatReporter.scala	73.91% <0%> (ø)	:arrow_up:
... and 17 more

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[benblack86]

You are correct that you can set the content-type via withHeader, but that isn't very useful since you'll end up with two content-type headers when you actually want one (the one set withHeader). I know the problem has bitten me twice and at least one other person. I'm not sure what functionality we would lose/break if content type was removed from the body.

[DanSimon]

Yeah that's definitely a good point. I'm going to go ahead and approve this PR. I'm still a little concerned that there's a better reason for keeping the header with the body that I'm just not thinking of right now, but since the PR still allows the HttpBodyEncoder to work properly, which was my main concern, I think it should be ok.

👍

[dxuhuang]

bump @benblack86 updated docs