mono/mono — pull request #474 by knocte (2012-10-03 11:10:32+00:00)
Title:  [System.XML]: Do not set XmlTypeMapElementInfo.IsNullable if it's already True

The IsNullable attribute in XmlElementAttribute is false by default,
so when not using it in a member which was already nullable
(i.e. Nullable), Mono was overriding the attribute causing
a FormatException later if an xsi:nil value was found when deserializing.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=7613