Unclear event semantics

[clue]

The Response is a ReadableStreamInterface, however it does not currently obey its event semantics:

• It will never emit a close event
• It will emit an end event if the underlying stream closes
• It will emit an end event with an error if the underlying stream ends

We should obey the stream semantics here, see also reactphp/stream#59.

Note that this will result in a BC break, so we should target the v0.5.0 release here. Even the examples (#69) highlight how the end event should actually be a close event instead.