Move `NilClass#to_h` to `mruby-object-ext` from `mruby-enum-ext`

mrbgems/mruby-enum-ext/mrblib/enum.rb

@@ -811,10 +811,6 @@ module Enumerable
     h
   end
 
-  def nil.to_h
-    {}
-  end
-
   def uniq(&block)
     hash = {}
     if block

mrbgems/mruby-enum-ext/test/enum.rb

@@ -186,8 +186,5 @@ assert("Enumerable#to_h") do
   h = c.new.to_h
   assert_equal Hash, h.class
   assert_equal h0, h
-  # mruby-enum-ext also provides nil.to_h
-  assert_equal Hash.new, nil.to_h
-
   assert_equal({1=>4,3=>8}, c.new.to_h{|k,v|[k,v*2]})
 end

mrbgems/mruby-object-ext/src/object.c

 #include <mruby.h>
 #include <mruby/array.h>
 #include <mruby/class.h>
+#include <mruby/hash.h>
 #include <mruby/proc.h>
 
 /*
@@ -31,6 +32,19 @@ nil_to_f(mrb_state *mrb, mrb_value obj)
 }
 #endif
 
+/*
+ *  call-seq:
+ *     nil.to_h    -> {}
+ *
+ *  Always returns an empty hash.
+ */
+
+static mrb_value
+nil_to_h(mrb_state *mrb, mrb_value obj)
+{
+  return mrb_hash_new(mrb);
+}
+
 /*
  *  call-seq:
  *     nil.to_i    -> 0
@@ -117,6 +131,7 @@ mrb_mruby_object_ext_gem_init(mrb_state* mrb)
 #ifndef MRB_WITHOUT_FLOAT
   mrb_define_method(mrb, n, "to_f", nil_to_f,       MRB_ARGS_NONE());
 #endif
+  mrb_define_method(mrb, n, "to_h", nil_to_h,       MRB_ARGS_NONE());
   mrb_define_method(mrb, n, "to_i", nil_to_i,       MRB_ARGS_NONE());
 
   mrb_define_method(mrb, mrb->kernel_module, "itself", mrb_f_itself, MRB_ARGS_NONE());

mrbgems/mruby-object-ext/test/nil.rb

@@ -7,6 +7,10 @@ assert('NilClass#to_f') do
   assert_equal 0.0, nil.to_f
 end
 
+assert('NilClass#to_h') do
+  assert_equal Hash.new, nil.to_h
+end
+
 assert('NilClass#to_i') do
   assert_equal 0, nil.to_i
 end