tests: Added new test for string/istream include interaction (this actually fails on DMC)

git-svn-id: http://pugixml.googlecode.com/svn/trunk@865 99668b35-9821-0410-8761-19e4c4f06640

tests/test_header_string_iostream.cpp

+// Tests compatibility with string/iostream
+#include <string>
+#include "../src/pugixml.hpp"
+#include <istream>
+#include <ostream>