Add support for missing cell reference (#268)

When describing a cell, the cell reference (r="A1") is optional.
When not present, we should just increment the index of the last processed row.

tests/Spout/Reader/XLSX/ReaderTest.php

@@ -140,6 +140,21 @@ class ReaderTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($expectedRows, $allRows);
     }
 
+    /**
+     * @return void
+     */
+    public function testReadShouldSupportFilesWithoutCellReference()
+    {
+        // file where the cell definition does not have a "r" attribute
+        // as in <c r="A1">...</c>
+        $allRows = $this->getAllRowsForFile('sheet_with_missing_cell_reference.xlsx');
+
+        $expectedRows = [
+            ['s1--A1'],
+        ];
+        $this->assertEquals($expectedRows, $allRows);
+    }
+
     /**
      * @return void
      */