hash.c: `Hash#shift` to return `nil` when a hash is empty.

It used to be return the default value if available, but it should
ignore the default value for behavior consistency. CRuby will adopt
this behavior too in the future. [ruby-bugs:16908]

src/hash.c

@@ -1506,7 +1506,7 @@ mrb_hash_shift(mrb_state *mrb, mrb_value hash)
 
   hash_modify(mrb, hash);
   if (h_size(h) == 0) {
-    return hash_default(mrb, hash, mrb_nil_value());
+    return mrb_nil_value();
   }
   else {
     mrb_value del_key, del_val;

test/t/hash.rb

@@ -775,7 +775,7 @@ assert('Hash#shift', '15.2.13.4.24') do
     assert_equal(0, h.size)
 
     h.default = -456
-    assert_equal(-456, h.shift)
+    assert_equal(nil, h.shift)
     assert_equal(0, h.size)
 
     h.freeze
@@ -783,8 +783,8 @@ assert('Hash#shift', '15.2.13.4.24') do
   end
 
   h = Hash.new{|h, k| [h, k]}
-  assert_operator(h.shift, :eql?, [h, nil])
   assert_equal(0, h.size)
+  assert_equal(nil, h.shift)
 end
 
 # Not ISO specified