GcTime incorrectly checks GC.respond_to?(:total_time), it should chec…

…k GC::Profiler.respond_to?(:total_time)

[#6435 state:committed]

Signed-off-by: Santiago Pastorino <santiago@wyeworks.com>

activesupport/lib/active_support/testing/performance.rb

@@ -401,7 +401,7 @@ class GcTime < Base
             Mode = RubyProf::GC_TIME if RubyProf.const_defined?(:GC_TIME)
 
             # Ruby 1.9 with GC::Profiler
-            if GC.respond_to?(:total_time)
+            if defined?(GC::Profiler) && GC::Profiler.respond_to?(:total_time)
               def measure
                 GC::Profiler.total_time
               end