Stop using NKF.guess to prevent misdetection

Refs. c556e9aa8a42622e25b8ffd3bcfd55512d1c6d88, #70

Conflicts:
	lib/jpmobile/util.rb
	spec/unit/receive_mail_spec.rb
	spec/unit/util_spec.rb
Prevent double-conversion

Conflicts:
	spec/unit/receive_mail_spec.rb