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/workbook/worksheet/selection.rb b/lib/axlsx/workbook/worksheet/selection.rb
index 556651e3..1a2633b4 100644
--- a/lib/axlsx/workbook/worksheet/selection.rb
+++ b/lib/axlsx/workbook/worksheet/selection.rb
@@ -74,7 +74,7 @@ module Axlsx
 
     # @see active_cell
     def active_cell=(v)
-      cell = (v.class == Axlsx::Cell ? v.r_abs : v)
+      cell = (v.instance_of?(Axlsx::Cell) ? v.r_abs : v)
       Axlsx::validate_string(cell)
       @active_cell = cell
     end