Remove reference to singleton_class, that's Ruby 1.9 only.

Guardfile

@@ -1,5 +1,5 @@
 guard 'rspec' do
   watch(%r{^spec/.+_spec\.rb})
-  watch(%r{^lib/(.+)\.rb})     { |m| "spec/lib/#{m[1]}_spec.rb" }
+  watch(%r{^lib/(.+)\.rb})     { |m| "spec/#{m[1]}_spec.rb" }
   watch('spec/spec_helper.rb') { "spec" }
 end

lib/hashie/extensions/indifferent_access.rb

@@ -38,7 +38,7 @@ module Hashie
       # a hash without modifying the actual class. This is what
       # allows IndifferentAccess to spread to sub-hashes.
       def self.inject!(hash)
-        hash.singleton_class.send :include, Hashie::Extensions::IndifferentAccess
+        (class << hash; self; end).send :include, Hashie::Extensions::IndifferentAccess
         hash.convert!
       end
 
@@ -103,7 +103,7 @@ module Hashie
       def hash_with_indifference?(other)
         other.is_a?(::Hash) &&
         other.respond_to?(:indifferent_access?) &&
-        other.indifference_access?
+        other.indifferent_access?
       end
     end
   end

lib/hashie/extensions/merge_initializer.rb

@@ -16,7 +16,7 @@ module Hashie
     #
     module MergeInitializer
       def initialize(hash = {}, default = nil, &block)
-        super(default, &block)
+        default ? super(default) : super(&block)
         update(hash)
       end
     end