* gems/default_gems: Make the SPELL_CHECKERS constant as compatible as possible

lib/did_you_mean.rb

@@ -114,8 +114,28 @@ module DidYouMean
   correct_error NoMatchingPatternKeyError, PatternKeyNameChecker if defined?(::NoMatchingPatternKeyError)
 
   # TODO: Remove on 3.3:
-  SPELL_CHECKERS = @spell_checkers
+  class DeprecatedMapping # :nodoc:
+    def []=(key, value)
+      warn "Calling `DidYouMean::SPELL_CHECKERS[#{key.to_s}] = #{value.to_s}' has been deprecated. " \
+           "Please call `DidYouMean.correct_error(#{key.to_s}, #{value.to_s})' instead."
+
+      DidYouMean.correct_error(key, value)
+    end
+
+    def merge!(hash)
+      warn "Calling `DidYouMean::SPELL_CHECKERS.merge!(error_name => spell_checker)' has been deprecated. " \
+           "Please call `DidYouMean.correct_error(error_name, spell_checker)' instead."
+
+      hash.each do |error_class, spell_checker|
+        DidYouMean.correct_error(error_class, spell_checker)
+      end
+    end
+  end
+
+  # TODO: Remove on 3.3:
+  SPELL_CHECKERS = DeprecatedMapping.new
   deprecate_constant :SPELL_CHECKERS
+  private_constant :DeprecatedMapping
 
   # Returns the currently set formatter. By default, it is set to +DidYouMean::Formatter+.
   def self.formatter