goss serve: render response format according to content negotiation

[petemounce]

Describe the feature:
goss serve can respond to the Accept http header on incoming requests, and choose the Content-Type for the response if possible based on that.

This will allow goss serve more flexibility at run-time; previously, if more than one output format was wanted, one endpoint process+port per output format would need to be set up.

Describe the solution you'd like

(This is a retroactive ticket to collect together the PRs as much as describe the work)

This is the order to merge the PRs in:

1. Add some test coverage to the goss serve command. #606 - add some unit test coverage
2. Add blackbox tests for serve command #608 - add some integration test coverage
3. goss serve can now negotiate response's content-type via accept request header #609 - add the content negotiation (with documentation)
   • this is a stacked PR on top of Add some test coverage to the goss serve command. #606 and should merge cleanly.

(Afterwards, #586, but since #608 adds a new script, I'll fix that up inside that PR once #608 is merged)

Describe alternatives you've considered
Setting up one endpoint per desired goss format.

[petemounce]

@aelsabbahy this issue acts as a "main entry point" for the cluster of PRs I've made; it overrides what I said about order in #609 (comment) and also answers your question in #608 (comment).

[aelsabbahy]

Didn't forget about this, just fell behind this weeekend on this. I'll try my best to review/merge all this before wed.

[aelsabbahy]

This is the order to merge the PRs in:

#606 - add some unit test coverage
#608 - add some integration test coverage
#609 - add the content negotiation (with documentation)
this is a stacked PR on top of #606 and should merge cleanly.
(Afterwards, #586, but since #608 adds a new script, I'll fix that up inside that PR once #608 is merged)

606 done/merged
608 doesn't fail build when tests fail, I don't think that was intentional (I've commented on the PR)
609 needs a rebase

[aelsabbahy]

Everything is merged except #609

It's currently failing the build, once that's green/merged this ticket can be closed

[petemounce]

@aelsabbahy thanks! I've fixed #609; I'd left in a deliberate test failure to debug a thing :(