Fix bug with empty strings in repeated string callbacks.

Fix suggested by Henrik Carlgren. Added also unit test for the bug.

Update issue 73
Status: FixedInGit

tests/callbacks.proto

@@ -11,5 +11,6 @@ message TestMessage {
     repeated fixed32 fixed32value = 3;
     repeated fixed64 fixed64value = 4;
     optional SubMessage submsg = 5;
+    repeated string repeatedstring = 6;
 }