Reduce object allocations in ActiveSupport::Subscriber

Freezing the string literal reduces the number of objects allocated
during integration test runs.

AllocationTracer before:
```
[["/rack/lib/rack/utils.rb", 499, :T_STRING], [50939, 2737, 78553, 0, 13, 1783920]]
[["/rails/activesupport/lib/active_support/subscriber.rb",
99, :T_STRING], [44959, 0, 42633, 0, 2, 1670800]]
[["/rack/lib/rack/utils.rb", 662, :T_STRING], [32979, 0, 31263, 0, 1, 1226009]]
[["/rack/lib/rack/body_proxy.rb", 34, :T_ARRAY], [29988, 0, 28438, 0, 1, 1114320]]
[["/rails/activesupport/lib/active_support/callbacks.rb", 165, :T_DATA], [29968, 0, 28422, 0, 2, 3675936]]
```

AllocationTracer after:
```
[["/rack/lib/rack/utils.rb", 499, :T_STRING], [50983, 4080, 67269, 0, 12, 1815600]]
[["/rack/lib/rack/utils.rb", 662, :T_STRING], [33001, 0, 28420, 0, 2, 1230809]]
[["/rack/lib/rack/body_proxy.rb", 34, :T_ARRAY], [29990, 0, 25812, 0, 1, 1118000]]
[["/rails/activesupport/lib/active_support/subscriber.rb", 99, :T_STRING], [29986, 0, 25803, 0, 1, 1117840]]
[["/rails/activesupport/lib/active_support/notifications/instrumenter.rb", 52, :T_HASH], [29984, 28, 26133, 0, 11, 4953280]]

1 files changed, 1 insertions, 1 deletions

diff --git a/activesupport/lib/active_support/subscriber.rb b/activesupport/lib/active_support/subscriber.rb
index 98be78b41b..cd40284660 100644
--- a/activesupport/lib/active_support/subscriber.rb
+++ b/activesupport/lib/active_support/subscriber.rb
@@ -96,7 +96,7 @@ module ActiveSupport
       event.end = finished
       event.payload.merge!(payload)
 
-      method = name.split('.').first
+      method = name.split('.'.freeze).first
       send(method, event)
     end