Add auto_filter defined name in Axlsx::Worksheet#to_xml_string (refs …

…randym#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.

lib/axlsx/workbook/worksheet/worksheet.rb

@@ -367,7 +367,6 @@ def name=(name)
     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 @@ def to_sheet_node_xml_string(str='')
     # 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|

test/workbook/worksheet/tc_worksheet.rb

@@ -514,7 +514,18 @@ def test_auto_filter
     assert_raise(ArgumentError) { @ws.auto_filter = 123 }
     @ws.auto_filter.range = "A1:D9"
     assert_equal(@ws.auto_filter.range, "A1:D9")
+    @ws.to_xml_string
+    assert(@wb.defined_names.any?{|defined_name| defined_name.name=='_xlnm._FilterDatabase'})
+  end
+
+  def test_auto_filter_assign
+    assert(@ws.auto_filter.range.nil?)
     assert(@wb.defined_names.none?{|defined_name| defined_name.name=='_xlnm._FilterDatabase'})
+    assert_raise(ArgumentError) { @ws.auto_filter = 123 }
+    @ws.auto_filter = "A1:D9"
+    assert_equal(@ws.auto_filter.range, "A1:D9")
+    @ws.to_xml_string
+    assert(@wb.defined_names.any?{|defined_name| defined_name.name=='_xlnm._FilterDatabase'})
   end
 
   def test_sheet_pr_for_auto_filter