Avoid unnecessary string creation/concatination for numeric conversion, use Double.toString() instead

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1595047 13f79535-47bb-0310-9956-ffa450edef68

src/ooxml/java/org/apache/poi/xssf/usermodel/XSSFCell.java

@@ -806,7 +806,7 @@ public final class XSSFCell implements Cell {
                     DateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy");
                     return sdf.format(getDateCellValue());
                 }
-                return getNumericCellValue() + "";
+                return Double.toString(getNumericCellValue());
             case CELL_TYPE_STRING:
                 return getRichStringCellValue().toString();
             default: