Inline strings can contain multiple value nodes

We were working under the assumption that XLSX's inline strings only had a single value node (`<t>`). This is incorrect.
To get the actual value of an inline string node, we need to concatenate the value of all its child nodes.

tests/Spout/Reader/XLSX/ReaderTest.php

@@ -72,6 +72,19 @@ class ReaderTest extends TestCase
         }
     }
 
+    /**
+     * @return void
+     */
+    public function testReadShouldSupportInlineStringsWithMultipleValueNodes()
+    {
+        $allRows = $this->getAllRowsForFile('sheet_with_multiple_value_nodes_in_inline_strings.xlsx');
+
+        $expectedRows = [
+            ['VALUE 1 VALUE 2 VALUE 3 VALUE 4', 's1 - B1'],
+        ];
+        $this->assertEquals($expectedRows, $allRows);
+    }
+
     /**
      * @return void
      */