Generate correct number of codes in BatchBuilder

Previously, due to a bug in the inner loop, BatchBuilder could generate
more codes than were requested.

This removes the inner loop in favour of a single, smarter outer loop.
Instead of always creating exactly batch_size new codes, we now create
up to batch_size new codes. On each loop we try to generate batch_size
new potential codes and only save the ones which are unique.

This should result in fewer selects in scenarios where there is
contention over codes. Though this is entire system is not well suited
for there to be many conflicts.

core/app/models/spree/promotion_code/batch_builder.rb

@@ -26,16 +26,13 @@ def build_promotion_codes
   private
 
   def generate_random_codes
-    total_batches = (number_of_codes.to_f / self.class.batch_size).ceil
+    created_codes = 0
 
-    total_batches.times do |i|
-      codes_for_current_batch = Set.new
-      codes_to_generate = [self.class.batch_size, number_of_codes - i * batch_size].min
+    while created_codes < number_of_codes
+      max_codes_to_generate = [self.class.batch_size, number_of_codes - created_codes].min
 
-      while codes_for_current_batch.size < codes_to_generate
-        new_codes = Array.new(codes_to_generate) { generate_random_code }.to_set
-        codes_for_current_batch += get_unique_codes(new_codes)
-      end
+      new_codes = Array.new(max_codes_to_generate) { generate_random_code }.uniq
+      codes_for_current_batch = get_unique_codes(new_codes)
 
       codes_for_current_batch.each do |value|
         Spree::PromotionCode.create!(
@@ -44,6 +41,7 @@ def generate_random_codes
           promotion_code_batch: promotion_code_batch
         )
       end
+      created_codes += codes_for_current_batch.size
     end
   end