Refactor random char generation

Fix the following safe offenses:
- Style/OperatorMethodCall
- Style/RandomWithOffset

And use Array.new to avoid an extra allocation:

```
Comparison (IPS):
Array.new(8) { rand(65..89).chr }.join:   492433.7 i/s
(0...8).map { 65.+(rand(25)).chr }.join:   432155.8 i/s - 1.14x  (± 0.00) slower

Comparison (Memory):
Array.new(8) { rand(65..89).chr }.join:        440 allocated
(0...8).map { 65.+(rand(25)).chr }.join:        560 allocated - 1.27x more
```

2 files changed, 1 insertions, 11 deletions

diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
index b68f9b61..1af7d16e 100644
--- a/.rubocop_todo.yml
+++ b/.rubocop_todo.yml
@@ -248,11 +248,6 @@ Style/NumericPredicate:
     - 'lib/axlsx/util/validators.rb'
     - 'lib/axlsx/workbook/worksheet/worksheet.rb'
 
-# This cop supports safe autocorrection (--autocorrect).
-Style/OperatorMethodCall:
-  Exclude:
-    - 'lib/axlsx/drawing/vml_shape.rb'
-
 # Configuration parameters: AllowedMethods.
 # AllowedMethods: respond_to_missing?
 Style/OptionalBooleanParameter:
@@ -319,11 +314,6 @@ Style/QuotedSymbols:
     - 'lib/axlsx/workbook/worksheet/worksheet_hyperlink.rb'
 
 # This cop supports safe autocorrection (--autocorrect).
-Style/RandomWithOffset:
-  Exclude:
-    - 'lib/axlsx/drawing/vml_shape.rb'
-
-# This cop supports safe autocorrection (--autocorrect).
 Style/RedundantRegexpEscape:
   Exclude:
     - 'lib/axlsx/workbook/worksheet/pivot_table.rb'
diff --git a/lib/axlsx/drawing/vml_shape.rb b/lib/axlsx/drawing/vml_shape.rb
index f1cd8951..4b21528f 100644
--- a/lib/axlsx/drawing/vml_shape.rb
+++ b/lib/axlsx/drawing/vml_shape.rb
@@ -24,7 +24,7 @@ module Axlsx
       @right_offset = 50
       @bottom_offset = 5
       @visible = true
-      @id = (0...8).map { 65.+(rand(25)).chr }.join
+      @id = Array.new(8) { rand(65..89).chr }.join
       parse_options options
       yield self if block_given?
     end