Fix non-const version of Common::String::begin.

Common::String::begin now assures the storage is a unique one, i.e. there are
no other Common::String objects pointing at it. This allows for safe use of
the writable iterators (and thus fixes the test case added with my last commit)

svn-id: r49323

common/str.h

@@ -222,6 +222,12 @@ public:
 	typedef const char *  const_iterator;
 
 	iterator begin() {
+		// Since the user could potentionally
+		// change the string via the returned
+		// iterator we have to assure we are
+		// pointing to an unique storage.
+		makeUnique();
+
 		return _str;
 	}