The canonical form of a filter: both parts trimmed, the attribute lower-cased
(the register's attributes are lower case), and the value in the register's
spelling (canonicalFilterValue: TRUE → true, FOI_EU_LAWS →
FOI_EUROPEAN_UNION|FOI_EU_LAWS). Idempotent. Parts that are not strings are left
for the checks in filterQuery / parseFilter to reject.
The canonical form of a filter: both parts trimmed, the attribute lower-cased (the register's attributes are lower case), and the value in the register's spelling (
canonicalFilterValue:TRUE→true,FOI_EU_LAWS→FOI_EUROPEAN_UNION|FOI_EU_LAWS). Idempotent. Parts that are not strings are left for the checks infilterQuery/parseFilterto reject.