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/Wrapper/XMLReader.php

@@ -164,4 +164,12 @@ class XMLReader extends \XMLReader
 
         return ($this->nodeType === $nodeType && $currentNodeName === $nodeName);
     }
+
+    /**
+     * @return string The name of the current node, un-prefixed
+     */
+    public function getCurrentNodeName()
+    {
+        return $this->localName;
+    }
 }