Do fewer intermediary allocations when calculating test methods (#89)

Showed up as the number one hotspot when profile the rubygems.org `rails test` startup time

lib/shoulda/context/context.rb

@@ -100,7 +100,7 @@ def test_unit_class
 
       def test_methods
         @test_methods ||= Hash.new { |h,k|
-          h[k] = Hash[k.instance_methods.map { |n| [n, true] }]
+          h[k] = k.instance_methods.each_with_object({}) { |n, a| a[n] = true }
         }
       end
 
@@ -213,4 +213,3 @@ def method_missing(method, *args, &blk)
     class DuplicateTestError < RuntimeError; end
   end
 end
-