uim-anthy (, uim-canna and possibly uim-prime) commits fullwidth space in
hankaku-katakana mode. This is caused by ja-direct in japanese.scm, but I can't
fix this because I don't know what is proper code for ja-direct. See comments of
ja-direct-rule in japanese.scm (copied below).

;; What's intended? The name "ja-direct-rule" makes me confused
;; because it acts in non-direct modes. This rule makes
;; uim-anthy improper as always sending zenkaku-space in
;; hanakaku-kana mode. This seems to be used to "always commit
;; zenkaku-space regardless input mode". If it's true, This rule
;; should be able to be disabled. -- YamaKen
(define ja-direct-rule
  '((" " "　")
    ))