Adds `Server::http1_header_read_timeout(Duration)`. Setting a duration will determine how long a client has to finish sending all the request headers before trigger a timeout test. This can help reduce resource usage when bad actors open connections without sending full requests.

Closes #2457

…2706)

Previous behavior returned a 404 Bad Request. Conforms to HTTP 1.1 RFC.

Closes #2701

…#2709)

According to rfc2616#section-14.20 the header value is case-insensitive. Certain clients send the expectation as `100-Continue` and this should be handled by the server.

Closes #2708

…mpty (#2715)

An HTTP/2 stream may include a set of headers, and a flag signalling
END-STREAM, even if a `content-length` isn't included. hyper wouldn't
notice, and so the `Body` would report a size-hint of `0..MAX`. hyper
now notices that the stream is ended, and couldn't possibly include any
bytes for the body, and thus will give a size-hint of `0` exactly.

This value is like a high-water mark. It applies per stream. Once a
stream has buffered that amount of bytes to send, it won't poll more
data from the `HttpBody` until the stream has been able to flush under
it.