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.

src/Spout/Reader/XLSX/Helper/CellValueFormatter.php

@@ -122,10 +122,15 @@ class CellValueFormatter
      */
     protected function formatInlineStringCellValue($node)
     {
-        // inline strings are formatted this way:
-        // <c r="A1" t="inlineStr"><is><t>[INLINE_STRING]</t></is></c>
-        $tNode = $node->getElementsByTagName(self::XML_NODE_INLINE_STRING_VALUE)->item(0);
-        $cellValue = $this->escaper->unescape($tNode->nodeValue);
+        // inline strings are formatted this way (they can contain any number of <t> nodes):
+        // <c r="A1" t="inlineStr"><is><t>[INLINE_STRING]</t><t>[INLINE_STRING_2]</t></is></c>
+        $tNodes = $node->getElementsByTagName(self::XML_NODE_INLINE_STRING_VALUE);
+
+        $cellValue = '';
+        for ($i = 0; $i < $tNodes->count(); $i++) {
+            $tNode = $tNodes->item($i);
+            $cellValue .= $this->escaper->unescape($tNode->nodeValue);
+        }
 
         return $cellValue;
     }