Add auto_filter defined name in `Axlsx::Worksheet#to_xml_string` (refs #285)

The way ot was done previously (adding the defined name on
`Axlsx::Worksheet#auto_filter=`) meant that it was added only when
using this method. But it's possible to assign range on the auto-created
`AutoFilter` with:

```ruby
workheet.auto_filter.range = "A1:B2"
```

In this case the defined name was never added to the workbook.

1 files changed, 4 insertions, 2 deletions

diff --git a/lib/axlsx/workbook/worksheet/worksheet.rb b/lib/axlsx/workbook/worksheet/worksheet.rb
index f8f2ff83..fcd883e1 100644
--- a/lib/axlsx/workbook/worksheet/worksheet.rb
+++ b/lib/axlsx/workbook/worksheet/worksheet.rb
@@ -367,7 +367,6 @@ module Axlsx
     def auto_filter=(v)
       DataTypeValidator.validate "Worksheet.auto_filter", String, v
       auto_filter.range = v
-      workbook.add_defined_name auto_filter.defined_name, name: '_xlnm._FilterDatabase', local_sheet_id: index, hidden: 1
     end
 
     # Accessor for controlling whether leading and trailing spaces in cells are
@@ -609,7 +608,10 @@ module Axlsx
     # This intentionally does not use nokogiri for performance reasons
     # @return [String]
     def to_xml_string
-      auto_filter.apply if auto_filter.range
+      if auto_filter.range
+        auto_filter.apply
+        workbook.add_defined_name auto_filter.defined_name, name: '_xlnm._FilterDatabase', local_sheet_id: index, hidden: 1
+      end
       str = '<?xml version="1.0" encoding="UTF-8"?>'
       str << worksheet_node
       serializable_parts.each do |item|