Fix for issue-41

Adding a validation to
NUnit.Framework.Internal.TextMessageWriter.WriteMessageLine(Int32,
String, params Object[]) to replace any embedded zeroes.
Fix for issue-41

- Removing validation added and instead now using already implemented
NUnit.Framework.Constraints.MsgUtils.EscapeControlChars(String) wich
also validates for others than zeroes.
- Adding two new test methods to
NUnit.Framework.Internal.TextMessageWriterTests for embedded zeroes.
Merge pull request #127 from elbaloo/issue-41

Fix: check for zeroes in assert messages #41