Fix safe class comparison offenses

Fixes:
- Style/ClassCheck
- Style/ClassEqualityComparison

While `is_a?` and `kind_of?` are equivalent, there is a 5% gain
when switching from class comparison to `intance_of?` 

```
Comparison:
         instance_of: 16902635.9 i/s
    class comparison: 16061288.3 i/s - 1.05x  (± 0.00) slower
```

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/axlsx/drawing/axes.rb b/lib/axlsx/drawing/axes.rb
index 6a74c879..010fe8cd 100644
--- a/lib/axlsx/drawing/axes.rb
+++ b/lib/axlsx/drawing/axes.rb
@@ -32,7 +32,7 @@ module Axlsx
     def to_xml_string(str = +'', options = {})
       if options[:ids]
         # CatAxis must come first in the XML (for Microsoft Excel at least)
-        sorted = axes.sort_by { |name, axis| axis.kind_of?(CatAxis) ? 0 : 1 }
+        sorted = axes.sort_by { |name, axis| axis.is_a?(CatAxis) ? 0 : 1 }
         sorted.each { |axis| str << '<c:axId val="' << axis[1].id.to_s << '"/>' }
       else
         axes.each { |axis| axis[1].to_xml_string(str) }