reference: allow more than 1 digest algorithm separator
This updates the grammar to allow more than one digest algorithm
separator, matching the regular expression and intended grammar.
// tag := /[\w][\w.-]{0,127}/
//
// digest := digest-algorithm ":" digest-hex
-// digest-algorithm := digest-algorithm-component [ digest-algorithm-separator digest-algorithm-component ]
+// digest-algorithm := digest-algorithm-component [ digest-algorithm-separator digest-algorithm-component ]*
// digest-algorithm-separator := /[+.-_]/
// digest-algorithm-component := /[A-Za-z][A-Za-z0-9]*/
// digest-hex := /[0-9a-fA-F]{32,}/ ; At least 128 bit digest value
