caddyhttp: Set Content-Type for static response

[mholt]

Since the Body field is a string, and it is generally configured via command-line or text config file, it follows that a reasonable default Content-Type is text/plain. This allows templates to render templates included in the response, and makes it clear to clients that they do not need to download a binary file, without extra configuration of this header.

We also detect valid JSON (JSON can be a number or quoted string which is easily ambiguous as to Content-Type, but the most common form of JSON is an object or array, so we only do it for those structures) and set application/json. But those are the only 2 special cases where we set the header automatically. I think those are probably the most common use cases for respond: plain text and hard-coded JSON.

This should make using the static_response handler less cumbersome and more convenient, especially now that it has a CLI shortcut with caddy respond.

[francislavoie]

Worth adding, in addition to enabling templates support, it'll also enable encode support since it also matches on the response content type header to determine if it should do its work.

[mholt]

True, as long as the body is the minimum length for encoding (I think 512 bytes currently), this should enable compression if that's configured!