[StringIndexer] Fix compare bug and moved strToIndex to private

strToIndex is dangerous because it can increment the use of a string, or
create a new string. This method should only be called by IndexedString.

tests/string-indexer-tests/string-indexer-tests.ino

@@ -62,6 +62,14 @@ test(indexer_same_strings_should_equal)
 	assertTrue(one == two);
 }
 
+test(indexer_compare_strings_with_same_beginning)
+{
+  IndexedString two("one_two");
+  IndexedString one("one");
+
+  assertNotEqual(one.getIndex(), two.getIndex());
+}
+
 test(indexer_indexed_operator_eq)
 {
 	IndexedString one("one");