Allow overriding `content-type` after `set_body_{bytes,json,string}`

The call to insert_header does nothing here:

let followed_page = ResponseTemplate::new(200)
	.set_body_string(followed_page_html)
	.insert_header("content-type", "text/html");

and instead requires doing this:

let followed_page = ResponseTemplate::new(200)
	.set_body_raw(followed_page_html.as_bytes(), "text/html; charset=utf-8");

I think this is confusing. Defaults are fine I guess, but it's weird that I can't set content-type to whatever I please to overwrite it.

Related

• https://github.com/LukeMathWalker/wiremock-rs/issues/54
• https://github.com/LukeMathWalker/wiremock-rs/issues/3

Start at the ResponseTemplate entry points set_body_bytes, set_body_json, set_body_string, and set_body_raw, then compare how they apply content-type with the behavior described in the related issues. Done means insert_header can replace the content-type set by the body helpers while their existing defaults continue to work.