fix visibility of Slop#method_missing
fix visibility of Slop#method_missing

Example:
    Object.new.respond_to?(:method_missing) #=> false
    Slop.new.respond_to?(:method_missing)   #=> true

NigoroJr/slop,prees1/slop,rickhull/slop,leejarvis/slop,brbrady/slop