Cast argument to string before performing regex

Recently we started getting warnings about calling #=~ on things like
booleans and arrays etc.

This commit ensures that we cast the argument to a string before
performing a regex on it to ensure that we aren't relying on what
calling a regex match type method on a boolean returns.

core/lib/spree/preferences/preferable.rb

@@ -158,8 +158,7 @@ def convert_preference_value(value, type)
           value.to_i
         when :boolean
           if !value ||
-             value == 0 ||
-             value =~ /\A(f|false|0)\Z/i ||
+             value.to_s =~ /\A(f|false|0|^)\Z/i ||
              (value.respond_to?(:empty?) && value.empty?)
             false
           else