Merge should return self

Library/Homebrew/build_environment.rb

@@ -9,6 +9,7 @@ class BuildEnvironment
 
   def merge(*args)
     @settings.merge(*args)
+    self
   end
 
   def <<(o)

Library/Homebrew/test/test_build_environment.rb

@@ -10,6 +10,10 @@ class BuildEnvironmentTests < Homebrew::TestCase
     assert_same @env, (@env << :foo)
   end
 
+  def test_merge_returns_self
+    assert_same @env, @env.merge([])
+  end
+
   def test_std?
     @env << :std
     assert_predicate @env, :std?