Making `preserve_header_order` option official

Is your feature request related to a problem? Please describe.

Our use case of Hyper is using it as a proxy server. One of the difference from proxy test we found is the order of the headers in the response. The order of the headers may be different in some cases, and we are seeing diff when comparing w/ and w/o Hyper in front of the existing server.

Describe the solution you'd like

Making preserve_header_order to official as it is now ffi.

https://github.com/hyperium/hyper/blob/master/src/client/conn/http1.rs#L473-L476

Describe alternatives you've considered

Use preserve_header_order with ffi enabled.

However, I'd like to understand if there is any blocking issue to make the feature to official.

If there is any, I can also help.

Additional context
Add any other context or screenshots about the feature request here.

Start in src/client/conn/http1.rs around lines 473-476 and trace how preserve_header_order is currently gated behind ffi. Determine the blockers to making it official, then verify that the option is available without ffi and preserves response header order for the proxy use case.