ComparableSet#merge returns self

Set#merge is supposed to return self, and Formula#recursive_requirements
depended on this being true.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/extend/set.rb

@@ -20,5 +20,6 @@ class ComparableSet < Set
   def merge enum
     enum.is_a?(Enumerable) or raise ArgumentError, "value must be enumerable"
     enum.each { |o| add(o) }
+    self
   end
 end

Library/Homebrew/test/test_comparableset.rb

@@ -33,7 +33,7 @@ class ComparableSetTests < Test::Unit::TestCase
     @set << X11Dependency.new
     @set << Requirement.new
     reqs = Set.new [X11Dependency.new('2.6'), Requirement.new]
-    @set.merge reqs
+    assert_same @set, @set.merge(reqs)
 
     assert_equal @set.count, 2
     assert_equal @set.find {|r| r.is_a? X11Dependency}, X11Dependency.new('2.6')