Alias ENV.merge! as ENV.update

[Feature #15947]

Closes: https://github.com/ruby/ruby/pull/2246

spec/ruby/core/env/shared/update.rb

@@ -0,0 +1,21 @@
+describe :env_update, shared: true do
+  it "adds the parameter hash to ENV" do
+    ENV["foo"].should == nil
+    ENV.send @method, "foo" => "bar"
+    ENV["foo"].should == "bar"
+    ENV.delete "foo"
+  end
+
+  it "yields key, the old value and the new value when replacing entries" do
+    ENV.send @method, "foo" => "bar"
+    ENV["foo"].should == "bar"
+    ENV.send(@method, "foo" => "boo") do |key, old, new|
+      key.should == "foo"
+      old.should == "bar"
+      new.should == "boo"
+      "rab"
+    end
+    ENV["foo"].should == "rab"
+    ENV.delete "foo"
+  end
+end