trillium-http-v0.3.8

What's new?

• 17062a0 add serialization for Headers with existing serde flag
• bfdbbfa expose copy under unstable flag
• f066c14 add Headers::try_insert_with and use it to lazily populate date header
• 1bdd4d6 BoxedTransport: Add methods to downcast to a specific transport type
• 9a914bb Add methods to get a Conn's Transport as a reference or mutable reference