Fix special characters in table header

[rfdonnelly]

Previous to this PR, special characters in table headers would generate invalid XML due to the use of the cell.value instead of cell.clean_value in table serialization.

This PR adds a test and fix for this.

Before the fix, the new test failed with:

1:0: ERROR: Element '{http://schemas.openxmlformats.org/spreadsheetml/2006/main}tableColumns': Character content other than whitespace is not allowed because the content type is 'element-only'.
F
===============================================================================
Failure: test_to_xml_string_for_special_characters(TestTable):
  error free validation.
  <false> is not true.
/home/rfdonnelly/repos/caxlsx/test/workbook/worksheet/tc_table.rb:80:in `test_to_xml_string_for_special_characters'
     77:       errors.push error
     78:       puts error.message
     79:     end
  => 80:     assert(errors.empty?, "error free validation")
     81:   end
     82: end
===============================================================================

After the fix, the new test passes.

[rfdonnelly]

I rewrote the test to match an existing special character test (TestSeriesTitle::test_to_xml_string_for_special_characters). Without the fix, the test now fails with:

Failure: test_to_xml_string_for_special_characters(TestTable):
  invalid xml: 1:262: FATAL: xmlParseEntityRef: no name, 1:263: FATAL: Unescaped '<' not allowed in attributes values, 1:263: FATAL: attributes construct error, 1:263: FATAL: Couldn't find end of Start Tag tableColumn line 1, 1:264: FATAL: StartTag: invalid element name.
  <false> is not true.
/home/rfdonnelly/repos/caxlsx/test/workbook/worksheet/tc_table.rb:75:in `test_to_xml_string_for_special_characters'
     72:     table = @ws.add_table("A1:D5")
     73:     doc = Nokogiri::XML(table.to_xml_string)
     74:     errors = doc.errors
  => 75:     assert(errors.empty?, "invalid xml: #{errors.map(&:to_s).join(', ')}")
     76:   end
     77: end

[kiskoza]

looks good to me, nice catch

[straydogstudio]

Thanks @rfdonnelly