perf(ext/http): optimize inferrable content-type responses

[littledivy]

Skip manually setting header via op_http_set_response_header if Content-Type can be inferred from body. Optimizes the case for new Response(<string>)

--strace-ops in main:

[     1.726] op_http_wait                                       : Dispatched Async
[     1.726] op_http_set_response_header                        : Dispatched Fast
[     1.726] op_http_set_response_header                        : Completed Fast
[     1.726] op_http_set_response_body_text                     : Dispatched Fast
[     1.726] op_http_set_response_body_text                     : Completed Fast
[     1.726] op_run_microtasks                                  : Completed Fast

--strace-ops for this patch:

[     1.726] op_http_wait                                       : Dispatched Async
[     1.726] op_http_set_response_body_text                     : Dispatched Fast
[     1.726] op_http_set_response_body_text                     : Completed Fast
[     1.726] op_run_microtasks                                  : Completed Fast

[lucacasonato]

This changes the behaviour of this code, and also changes the behaviour of Deno.serve with a response returned that was created like this.

> const resp = new Response("hello world")
undefined
> resp.headers
Headers { "content-type": "text/plain;charset=UTF-8" }
> resp.headers.delete("content-type")
undefined
> resp.headers
Headers {}

[littledivy]

@lucacasonato Oh that's interesting, maybe we can disable optimization when delete("content-type"). Are there any other cases like this?

[lucacasonato]

If I am reading your code correctly, resp.headers right after new Response("hello world") would be empty. We should fix that too (or I am misreading the code). I misread the code.

[lucacasonato]

I can't think of other cases right now.