Revert "hash merging with a block ignores non-existing keys altogether"

This reverts commit 72f9fec6.

It is wrong, this feature is a little undocumented, doing some research.

activesupport/lib/active_support/ordered_hash.rb

@@ -131,7 +131,7 @@ def shift
 
       def merge!(other_hash)
         if block_given?
-          other_hash.each {|k,v| self[k] = yield(k, self[k], v) if key? k}
+          other_hash.each {|k,v| self[k] = yield(k, self[k], v) }
         else
           other_hash.each {|k,v| self[k] = v }
         end

activesupport/test/ordered_hash_test.rb

@@ -144,27 +144,28 @@ def test_merge
   end
 
   def test_merge_with_block
-    hash = ActiveSupport::OrderedHash.new
-    hash[:a] = 0
-    hash[:b] = 0
-    merged = hash.merge(:b => 1, :c => 2) do |key, old_value, new_value|
-      new_value + 1
+    other_hash = ActiveSupport::OrderedHash.new
+    other_hash['white'] = 'ff'
+    other_hash['black'] = '00'
+    merged = @ordered_hash.merge(other_hash) do |key, old_value, new_value|
+      new_value * 3
     end
-    assert_equal 0, merged[:a]
-    assert_equal 2, merged[:b]
-    assert !merged.key?(:c)
+    assert_equal 'ffffff', merged['white']
+    assert_equal '000000', merged['black']
+
+    assert_nil @ordered_hash['white']
+    assert_nil @ordered_hash['black']
   end
 
   def test_merge_bang_with_block
-    hash = ActiveSupport::OrderedHash.new
-    hash[:a] = 0
-    hash[:b] = 0
-    hash.merge!(:a => 1, :c => 2) do |key, old_value, new_value|
-      new_value + 3
+    other_hash = ActiveSupport::OrderedHash.new
+    other_hash['white'] = 'ff'
+    other_hash['black'] = '00'
+    @ordered_hash.merge!(other_hash) do |key, old_value, new_value|
+      new_value * 3
     end
-    assert_equal 4, hash[:a]
-    assert_equal 0, hash[:b]
-    assert !hash.key?(:c)
+    assert_equal 'ffffff', @ordered_hash['white']
+    assert_equal '000000', @ordered_hash['black']
   end
 
   def test_shift