Don't discard source after rendering

Previously, we would discard the template source after rendering, if we
had a virtual path, in hopes that the virtual path would let us find our
same template again going through the Resolver.

Previously we discarded the source as an optimization, to avoid keeping
it around in memory. By instead just reading the file every time source
is called, as FileTemplate does, this is unnecessary.

actionview/lib/action_view/template.rb

@@ -298,9 +298,6 @@ module ActionView
             compile(mod)
           end
 
-          # Just discard the source if we have a virtual path. This
-          # means we can get the template back.
-          @source = nil if @virtual_path
           @compiled = true
         end
       end

actionview/test/template/template_test.rb

@@ -91,10 +91,10 @@ class TestERBTemplate < ActiveSupport::TestCase
     assert_equal "<%= hello %>", render
   end
 
-  def test_template_loses_its_source_after_rendering
+  def test_template_does_not_lose_its_source_after_rendering
     @template = new_template
     render
-    assert_nil @template.source
+    assert_equal "<%= hello %>", @template.source
   end
 
   def test_template_does_not_lose_its_source_after_rendering_if_it_does_not_have_a_virtual_path