Raise ArgumentError from TimeZone.iso8601 when invalid value can be parsed by Date._iso8601

Date._iso8601 will return a hash for some values eg. '12936' but this will not contain the expected :mon and :mday keys.
Check for these keys and raise an ArgumentError if they aren't present as this is consistent with other invalid input behaviour.

Author: steventux

activesupport/lib/active_support/values/time_zone.rb

@@ -383,8 +383,6 @@ def at(*args)
     def iso8601(str)
       parts = Date._iso8601(str)
 
-      raise ArgumentError, "invalid date" if parts.empty?
-
       time = Time.new(
         parts.fetch(:year),
         parts.fetch(:mon),
@@ -400,6 +398,9 @@ def iso8601(str)
       else
         TimeWithZone.new(nil, self, time)
       end
+
+    rescue KeyError
+      raise ArgumentError, "invalid date"
     end
 
     # Method for creating new ActiveSupport::TimeWithZone instance in time zone

activesupport/test/time_zone_test.rb

@@ -343,6 +343,16 @@ def test_iso8601_with_ambiguous_time
     assert_equal Time.utc(2014, 10, 25, 22, 0, 0), zone.parse("2014-10-26T01:00:00")
   end
 
+  def test_iso8601_with_invalid_value_parseable_by_date__iso8601
+    zone = ActiveSupport::TimeZone["Eastern Time (US & Canada)"]
+
+    exception = assert_raises(ArgumentError) do
+      zone.iso8601("12936")
+    end
+
+    assert_equal "invalid date", exception.message
+  end
+
   def test_parse
     zone = ActiveSupport::TimeZone["Eastern Time (US & Canada)"]
     twz = zone.parse("1999-12-31 19:00:00")