HTTP client: Support for expect-continue (`Expect: 100-continue`)

[clue]

Self-quoting from our server-side implementation: https://github.com/reactphp/http/issues/109#issuecomment-278958298

Thanks for filing this ticket and sparking the discussion about our expect-continue feature (Expect: 100-continue) :+1:

First things first, here's how this works in the first place:

• This feature is specified as part of HTTP/1.1: https://tools.ietf.org/html/rfc7231#section-5.1.1
• Clients MAY implement this feature but are not required to do so
• Severs MAY implement this feature but are not required to do so
• If the client includes a Expect: 100-continue header, the server MAY return an intermediary HTTP/1.1 100 Continue response after receiving the request headers, which causes the client to start transmitting the actual message body
• If the client includes a Expect: 100-continue header, the server MAY reject the request with any "normal" error response message, which means the client never has to transmit the message body at all
• Clients MUST support servers that do not implement this feature. This usually adds a few seconds of delay where the client waits for the intermediary HTTP/1.1 100 Continue response
• Many common clients actually implement this feature when sending large requests only (usually a few KiB or MiB)

This means that it makes sense to automatically use this for certain requests, but we should probably provide an option to turn this off.