Improve performance of AbstractController::Base#action_methods

I was benchmarking some specs in my app, and saw this code come up in the
memory_profiler. It is by no means the biggest memory allocation, but it
is straightforward to make a slight improvement.

Primarily, this saves allocating one array by using concat instead of +
to add public_instance_methods(false).

That ends up being 10% less memory for my benchmark (3 actions), and 6%
faster.

```
Calculating -------------------------------------
            original     8.352k memsize (   208.000  retained)
                        22.000  objects (     2.000  retained)
                         6.000  strings (     0.000  retained)
         refactored3     7.616k memsize (   408.000  retained)
                        11.000  objects (     7.000  retained)
                         3.000  strings (     3.000  retained)

Comparison:
         refactored3:       7616 allocated
            original:       8352 allocated - 1.10x more
Warming up --------------------------------------
            original     2.326k i/100ms
         refactored3     2.441k i/100ms
Calculating -------------------------------------
            original     23.336k (± 0.7%) i/s -    118.626k in   5.083658s
         refactored3     24.692k (± 1.2%) i/s -    124.491k in   5.042345s

Comparison:
            original:    23336.0 i/s
         refactored3:    24692.5 i/s - 1.06x  faster
```

Benchmark and results are also posted to https://gist.github.com/technicalpickles/4a4ae6a9e2c42963af43a89f75e768fe

Author: technicalpickles

actionpack/lib/abstract_controller/base.rb

@@ -89,18 +89,15 @@ def internal_methods
       # ==== Returns
       # * <tt>Set</tt> - A set of all methods that should be considered actions.
       def action_methods
+        # All public instance methods of this class, including ancestors
         @action_methods ||= begin
-          # All public instance methods of this class, including ancestors
-          methods = (public_instance_methods(true) -
-            # Except for public instance methods of Base and its ancestors
-            internal_methods +
-            # Be sure to include shadowed public instance methods of this class
-            public_instance_methods(false))
-
-          methods.map!(&:to_s)
-
-          methods.to_set
-        end
+                              # Except for public instance methods of Base and its ancestors
+                              methods = public_instance_methods(true) - internal_methods
+                              # Except for public instance methods of Base and its ancestors
+                              methods.concat(public_instance_methods(false))
+                              methods.map!(&:to_s)
+                              methods.to_set
+                            end
       end
 
       # action_methods are cached and there is sometimes a need to refresh