Support XLSX with prefixed XML files (#237)

While the standard is not to have prefixes, some XLSX files have XML files containing a prefix.
Microsoft has a tool that generates such files: https://msdn.microsoft.com/en-us/library/office/gg278316.aspx

tests/Spout/Reader/XLSX/ReaderTest.php

@@ -95,6 +95,23 @@ class ReaderTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($expectedRows, $allRows);
     }
 
+    /**
+     * @return void
+     */
+    public function testReadShouldSupportPrefixedXMLFiles()
+    {
+        // The XML files of this spreadsheet are prefixed.
+        // For instance, they use "<x:sheet>" instead of "<sheet>", etc.
+        $allRows = $this->getAllRowsForFile('sheet_with_prefixed_xml_files.xlsx');
+
+        $expectedRows = [
+            ['s1 - A1', 's1 - B1', 's1 - C1'],
+            ['s1 - A2', 's1 - B2', 's1 - C2'],
+            ['s1 - A3', 's1 - B3', 's1 - C3'],
+        ];
+        $this->assertEquals($expectedRows, $allRows);
+    }
+
     /**
      * @return void
      */