check for zeroes in assert messages

[Peter Brightman]

[Issue now tracked at https://github.com/nunit/nunit-framework/issues/41]

It happened to us that somehow there was a zero in an assertion message. This zero within the message, even embedded into a cdata-section within the resultfile caused a crash to all applications that tried to create a DOM, .NET XmlDocument class and eben XmlSpy application crashed. Of course a zero is not allowed in a xml file but this zero came from the assertion message and so went into the NUnit resultfile. NUnit should check if messages are really valid strings and if there is a disallowed zero, escape it or convert it to its string representation.