• Add must_use to RequestBuilder and ClientBuilder.
• Fix Windows system proxy detection of Fiddler proxies.
• (wasm) Add headers method to RequestBuilder.
• (wasm) Add execute method to Client.
• (wasm) Add TryFrom<http::Request> for Request.
• (wasm) Fix checking for global window to work in non-browser environments.
• (wasm) Fix sending of an empty body when not required.