Clarify some handling about extension frame and unknown frame on HTTP/2

[蕭雋涵 <[e-mail]>]

It's related to https://tools.ietf.org/html/rfc7540#section-5.5 Extending
HTTP/2 and a test case for it
https://github.com/summerwind/h2spec/blob/master/5_5.go#L75-L102

The fourth paragraph said

> Implementations MUST ignore unknown or unsupported values in all
> extensible protocol elements. Implementations MUST discard frames
> that have unknown or unsupported types.


and it also said

> extension frames that appear in the middle of a header block are not
> permitted;
> these MUST be treated as a connection error of type PROTOCOL_ERROR.


So when one end point received one unknown frame in the middle of header
block,
I'm not sure what's the correct way to handle the frame with following two
options.
- the end point should drop the frame without any error
- the end point should close the connection with protocol error.

There is also one paragraph related to this that support the first option.
https://tools.ietf.org/html/rfc7540#section-5.1
at second from the last paragraph

> In the absence of more specific guidance elsewhere in this document,
> implementations SHOULD treat the receipt of a frame that is not
> expressly permitted in the description of a state as a connection
> error of type PROTOCOL_ERROR. Note that PRIORITY can
> be sent and received in any stream state. Frames of unknown types
> are ignored.

That said frames of unknown types should ignore and not treat as protocol
error.

So I think each the option make sense to me, and want to clarify for which
would be the best option.

Thanks