Fix prefixed shared strings XML file (#450)

A prefixed sharedStrings.xml file was not properly read, as we were comparing the un-prefixed name with the possible prefixed name.
Also, this commit contains a fix for sheets with rows not starting at column A.

src/Spout/Reader/XLSX/RowIterator.php

@@ -348,7 +348,7 @@ class RowIterator implements IteratorInterface
      */
     protected function isEmptyRow($rowData)
     {
-        return (count($rowData) === 1 && $rowData[0] === '');
+        return (count($rowData) === 1 && key($rowData) === '');
     }
 
     /**