Provide a way to force the queue writev strategy

[seanmonstar]

There is http1_writev(bool) currently to allow someone to force alwaysing using the "flatten" strategy of writing. However, when calling http1_writev(true), that just keeps the default, which is "auto". There should be a way for users to specify "always use the queue strategy, don't use auto".

The most logical way would be to keep "auto" as the default, calling http1_writev(false) would force "flatten", and calling http1_writev(true) would force "queue". This can be done by changing the builders to hold http1_writev: Option<bool> internally.