Fix matcher chaining to prevent name collisions.

Previously, #chain used #define_method on the class to define an instance method on all instances.  We want it to define the method on only this instance, so we need to delegate to the singleton class instead (which the recently added #define_method does for us!).

lib/rspec/matchers/matcher.rb

@@ -97,11 +97,9 @@ def diffable
 
       # See RSpec::Matchers
       def chain(method, &block)
-        self.class.class_eval do
-          define_method method do |*args|
-            block.call(*args)
-            self
-          end
+        define_method method do |*args|
+          block.call(*args)
+          self
         end
       end
 

spec/rspec/matchers/matcher_spec.rb

@@ -334,6 +334,14 @@ def assert_equal(a,b)
         matcher.expecting('value').matches?('other value').should be_false
       end
 
+      it "prevents name collisions on chainable methods from different matchers" do
+        m1 = RSpec::Matchers::Matcher.new(:m1) { chain(:foo) { raise "foo in m1" } }
+        m2 = RSpec::Matchers::Matcher.new(:m2) { chain(:foo) { raise "foo in m2" } }
+
+        expect { m1.foo }.to raise_error("foo in m1")
+        expect { m2.foo }.to raise_error("foo in m2")
+      end
+
       context "defined using the dsl" do
         def a_method_in_the_example
           "method defined in the example"