Make server_val productions more consistent.
Fix Require value list production.
Fix/test Accept media_range parsing.
Deduplicate DIGIT/SIP_Version/MIME_Version rules.
Collapse Method rule to: token
Collapse Status_Code rule to: _PDIGIT3
Collapse m_type, m_subtype rules to: token
Collapse disp_type rule to: token
Collapse substate_value rule to: token
Collapse event_reason_value rule to: token