Handle invalid DateTime attributes gracefully

The Date typecaster will gracefully handle invalid dates, but DateTime
does not. This changes the behavior of DateTime types to match the
behavior of Date types.

lib/active_remote/typecasting/date_time_typecaster.rb

@@ -3,6 +3,7 @@ module Typecasting
     class DateTimeTypecaster
       def self.call(value)
         value.to_datetime if value.respond_to?(:to_datetime)
+      rescue NoMethodError, ArgumentError
       end
     end
   end

spec/lib/active_remote/typecasting_spec.rb

@@ -14,6 +14,13 @@
       record = test_class.new(:birthday => "2016-01-01")
       expect(record.birthday).to eq(DateTime.parse("2016-01-01"))
     end
+
+    context "invalid date" do
+      it "sets attribute to nil" do
+        record = test_class.new(:birthday => "23451234")
+        expect(record.birthday).to be_nil
+      end
+    end
   end
 
   describe "float" do