Speed-up TimeWithZone to String conversion

I've noticed that `String(model.created_at)` is performing poorly in comparision
with other fields. The source of the problem is a way `Kernel#String` works: it first
tries to call `to_str` (which causes `NoMethodError` in `method_missing`) and then calls `to_s`.

Performance tests:

    tz = Time.zone.now

    Benchmark.ips do |x|
      x.report { String(tz) }
    end

Without this code:

    Calculating -------------------------------------
                               572 i/100ms
    -------------------------------------------------
                            10177.7 (±18.2%) i/s -      48620 in   5.000325s

With this code:

    Calculating -------------------------------------
                              1518 i/100ms
    -------------------------------------------------
                           138984.2 (±10.1%) i/s -     677028 in   4.974897s

1 files changed, 8 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/time_with_zone.rb b/activesupport/lib/active_support/time_with_zone.rb
index f2a2f3c3db..3155c0ab0f 100644
--- a/activesupport/lib/active_support/time_with_zone.rb
+++ b/activesupport/lib/active_support/time_with_zone.rb
@@ -353,6 +353,14 @@ module ActiveSupport
       initialize(variables[0].utc, ::Time.find_zone(variables[1]), variables[2].utc)
     end
 
+    # respond_to_missing? is not called in some cases, such as when type conversion is
+    # performed with Kernel#String
+    def respond_to?(sym, include_priv = false)
+      # ensure that we're not going to throw and rescue from NoMethodError in method_missing which is slow
+      return false if sym.to_sym == :to_str
+      super
+    end
+
     # Ensure proxy class responds to all methods that underlying time instance
     # responds to.
     def respond_to_missing?(sym, include_priv)