may/should

Hi,

Mark's recent mails about MAY/SHOULD/MUST reminded me to check for 
may/should/must as well; and there are quite a few places we 
should^h^h^h^h^h^hought to adjust.

For the trivial ones I applied 
<http://trac.tools.ietf.org/wg/httpbis/trac/changeset/1685>; for the 
remaining instances we might need to UPPERCASE them.

Best regards, Julian