Make sure that concurrent map usage is thread-safe

Behavior upon missing prefix partial name may cause a key to overwrite when executed in multiple threads at the same time.

ref ruby-concurrency/concurrent-ruby#970

Author: mensfeld

actionview/lib/action_view/renderer/abstract_renderer.rb

@@ -31,7 +31,7 @@ def render
 
     module ObjectRendering # :nodoc:
       PREFIXED_PARTIAL_NAMES = Concurrent::Map.new do |h, k|
-        h[k] = Concurrent::Map.new
+        h.compute_if_absent(k) { Concurrent::Map.new }
       end
 
       def initialize(lookup_context, options)