Fix Formula#<=> on trunk Ruby

Arguably this method shouldn't exist and sort_by(&:name) used instead.

Library/Homebrew/formula.rb

@@ -301,9 +301,12 @@ class Formula
   def hash
     name.hash
   end
-  def <=> b
-    name <=> b.name
+
+  def <=>(other)
+    return unless Formula === other
+    name <=> other.name
   end
+
   def to_s
     name
   end

Library/Homebrew/test/test_formula.rb

@@ -139,6 +139,10 @@ class FormulaTests < Homebrew::TestCase
     refute_equal TestBall.new, Object.new
   end
 
+  def test_sort_operator
+    assert_nil TestBall.new <=> Object.new
+  end
+
   def test_class_naming
     assert_equal 'ShellFm', Formulary.class_s('shell.fm')
     assert_equal 'Fooxx', Formulary.class_s('foo++')