Don't store @real_timezone if the instance has been frozen.

See #80.

lib/tzinfo/timezone_proxy.rb

@@ -82,7 +82,13 @@ def real_timezone
       # calculated multiple times in concurrently executing threads. It is not
       # worth the overhead of locking to ensure that @real_timezone is only
       # calculated once.
-      @real_timezone ||= Timezone.get(@identifier)
+      unless @real_timezone
+        result = Timezone.get(@identifier)
+        return result if frozen?
+        @real_timezone = result
+      end
+
+      @real_timezone
     end
   end
 end

test/tc_timezone_proxy.rb

@@ -102,6 +102,15 @@ def test_canonical_linked
     end
   end
 
+  def test_after_freeze
+    proxy = TimezoneProxy.new('Europe/London')
+    real = Timezone.get('Europe/London')
+    t = Time.utc(2017, 6, 1)
+    proxy.freeze
+    assert_equal('Europe/London', proxy.identifier)
+    assert_equal(real.utc_to_local(t), proxy.utc_to_local(t))
+  end
+
   def test_equals
     assert_equal(true, TimezoneProxy.new('Europe/London') == TimezoneProxy.new('Europe/London'))
     assert_equal(false, TimezoneProxy.new('Europe/London') == TimezoneProxy.new('Europe/Paris'))