Stash methods that get and set thread local variables to allow the us…

…er to mock them

Fixes #605.

lib/rspec/support.rb

@@ -90,15 +90,18 @@ def self.class_of(object)
       singleton_class.ancestors.find { |ancestor| !ancestor.equal?(singleton_class) }
     end
 
-    # A single thread local variable so we don't excessively pollute that namespace.
+    # Stash original methods to allow the user to mock them.
     if RUBY_VERSION.to_f >= 2
-      def self.thread_local_data
-        Thread.current.thread_variable_get(:__rspec) || Thread.current.thread_variable_set(:__rspec, {})
-      end
+      @thread_variable_get = Thread.instance_method(:thread_variable_get)
+      @thread_variable_set = Thread.instance_method(:thread_variable_set)
     else
-      def self.thread_local_data
-        Thread.current[:__rspec] ||= {}
-      end
+      @thread_variable_get = Thread.instance_method(:[])
+      @thread_variable_set = Thread.instance_method(:[]=)
+    end
+
+    # A single thread local variable so we don't excessively pollute that namespace.
+    def self.thread_local_data
+      @thread_variable_get.bind(Thread.current).call(:__rspec) || @thread_variable_set.bind(Thread.current).call(:__rspec, {})
     end
 
     # @api private