[Unencoded-Digest] Allow parameterized values. (#3132)

The spec is currently silent on the subject of parameters, and it seems
reasonable to consistently make room for future extension by explicitly
defining them as allowed but ignored. This has little practical impact,
but hopefully ensures that implementers would treat
`sha-256=:...:;options=plz` and `sha-256=:...:` as equivalent
declarations.

Author: mikewest

draft-ietf-httpbis-unencoded-digest.md

@@ -139,6 +139,10 @@ where each:
   conveys an encoded version of the byte output produced by the digest
   calculation.
 
+  Each Dictionary value can have zero or more Parameters ({{Section 3.1.2 of
+  STRUCTURED-FIELDS}}). This specification does not define any Parameters but
+  extensions MAY define them. Unknown Parameters MUST be ignored.
+
 For example:
 
 ~~~ http-message
@@ -202,6 +206,10 @@ constraints that operate in addition to this specification.
   inclusive. 1 is the least preferred, 10 is the most preferred, and a value of
   0 means "not acceptable".
 
+  Each Dictionary value can have zero or more Parameters ({{Section 3.1.2 of
+  STRUCTURED-FIELDS}}). This specification does not define any Parameters but
+  extensions MAY define them. Unknown Parameters MUST be ignored.
+
 Examples:
 
 ~~~ http-message