Floats must not be stored as locale dependent

Floats are currently stored formatted per the locale setting. This leads to different values being written whether the locale uses "." or "," for the decimal point for instance. This poses a problem as floats must be stored using "." as the decimal point to be valid.
This commit ensures that the floats are stored correctly by forcing the formatting of the value.

src/Spout/Writer/XLSX/Manager/WorksheetManager.php

@@ -240,7 +240,7 @@ EOD;
         } elseif ($cell->isBoolean()) {
             $cellXML .= ' t="b"><v>' . (int) ($cell->getValue()) . '</v></c>';
         } elseif ($cell->isNumeric()) {
-            $cellXML .= '><v>' . $cell->getValue() . '</v></c>';
+            $cellXML .= '><v>' . $this->stringHelper->formatNumericValue($cell->getValue()) . '</v></c>';
         } elseif ($cell->isError() && is_string($cell->getValueEvenIfError())) {
             // only writes the error value if it's a string
             $cellXML .= ' t="e"><v>' . $cell->getValueEvenIfError() . '</v></c>';