draft-ietf-httpbis-http2-16: use of "optional"

Hi there,

(now getting to the nitpicking...)

in several cases, the spec says "optional".

Some people in the IETF believe it's equivalent to "OPTIONAL", while 
others disagree. I recommend to uppercase "optional" where it's supposed 
to express a requirement, and to choose a different where it's not.

Best regards, Julian