* ws-client: Replace `httparse::Header` with `http::HeaderMap`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* ws-client: Make headers optional

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* http-client: Expose custom header injection

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* http-client: Adjust testing for custom headers

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Make `http::HeaderMap` non-optional

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* http-client: Cache request headers

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Fix doc tests

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* http-client: Use `into_iter` for headers

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* docs: Improve custom headers documentation

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* http: Use `hyper::http` instead of `http` directly

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* http-client: Adjust testing

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Fix doc tests

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* client: Expose `http::HeaderMap` and `http::HeaderValue`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>