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, 11 insertions, 0 deletions

diff --git a/test/workbook/worksheet/tc_worksheet.rb b/test/workbook/worksheet/tc_worksheet.rb
index 7cefbda9..74253613 100644
--- a/test/workbook/worksheet/tc_worksheet.rb
+++ b/test/workbook/worksheet/tc_worksheet.rb
@@ -514,7 +514,18 @@ class TestWorksheet < Test::Unit::TestCase
     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