Allow arbitrary header names

[passcod]

Currently the set and append header methods on Response take a static str for the key:

pub fn set_header(self, key: &'static str, value: impl AsRef<str>) -> Self;
pub fn append_header(self, key: &'static str, value: impl AsRef<str>) -> Self;

I'd like to be able to specify arbitrary header names:

pub fn set_header(self, key: &str, value: impl AsRef<str>) -> Self;
pub fn append_header(self, key: &str, value: impl AsRef<str>) -> Self;

Example usecases:

• echo server, copying (most) headers
• varying header name based on configuration
• (my immediate usecase) processing the request in a 'plugin'

[passcod]

For reference, my current workaround:

let mut res = tide::http::Response::builder();
res.status(meta.status);
for (key, value) in meta.headers {
    res.header(key.as_slice(), value.as_slice());
}

let res = tide::Response::from(res.body(http_service::Body::empty())?);

[Fishrock123]

This has been fixed since #414 (0.7.0). The header apis now use http_types::headers::HeaderName, which uses String internally.