fix: Handle missing cell references

[bf4]

Summary

• infer missing cell references from column order within a row
• guard empty-cell padding when row or last column is missing
• add regression fixture and spec for missing cell references

Testing

bundle exec rake

Closes #135

[stenlarsson]

@bf4 I also stumbled over this problem, but my fix is much simpler

diff --git a/lib/creek/sheet.rb b/lib/creek/sheet.rb
index 0c48fed..c6b5c86 100644
--- a/lib/creek/sheet.rb
+++ b/lib/creek/sheet.rb
@@ -120,6 +120,7 @@ module Creek
                 cell_type      = node.attributes['t']
                 cell_style_idx = node.attributes['s']
                 cell           = node.attributes['r']
+                cell ||= (cells.keys.last&.delete('0-9')&.succ || 'A') + row['r']
               elsif %w[v t].include?(node.name) && node.node_type == opener
                 unless cell.nil?
                   node.read

Your spec still passes. What do you think?