Added testing function for ZString.

IMFMessage.cpp

@@ -57,6 +57,10 @@ namespace coreutils {
       return ZString();
    }
 
+   ZString IMFMessage::getHeaderKeyPairValue(const char *headerKey, const char *key) {
+      return getHeaderKeyPairValue(ZString(headerKey), ZString(key));
+   }
+
    ZString IMFMessage::getHeaderKeyPairValue(ZString headerKey, ZString key) {
       ZString value;
       ZString psource(getHeader(headerKey, false));