Added assignment to MString from ZString operator=.

MString.cpp

@@ -80,6 +80,13 @@ namespace coreutils {
       length = value.getLength();
       return *this;
    }
+
+   MString &MString::operator=(coreutils::ZString value) {
+      setSize(value.getLength());
+      memcpy(data, value.getData(), value.getLength());
+      length = value.getLength();
+      return *this;
+   }
    
    MString &MString::operator=(const char *value) {
       int len = strlen(value);