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
```

1 files changed, 1 insertions, 1 deletions

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