Return the pop'ed Buffer from the BufferStack on flush_and_pop

lib/rackstash/buffer_stack.rb

@@ -54,11 +54,12 @@ module Rackstash
     # If there was a buffer on the stack and it has pending data, it is flushed
     # to the {#sink} before it is returned.
     #
-    # @return [nil]
+    # @return [Buffer, nil] the removed {Buffer} or `nil` if there was no
+    #   {Buffer} to remove
     def flush_and_pop
-      buffer = @stack_mutex.synchronize { @stack.pop }
+      buffer = @stack.pop
       buffer.flush if buffer
-      nil
+      buffer
     end
   end
 end

spec/rackstash/buffer_stack_spec.rb

@@ -70,9 +70,9 @@ describe Rackstash::BufferStack do
         .not_to change { stack.instance_variable_get(:@stack) }
     end
 
-    it 'always returns nil' do
+    it 'returns the pop\'ed Buffer' do
       stack.push
-      expect(stack.flush_and_pop).to be nil
+      expect(stack.flush_and_pop).to be_a Rackstash::Buffer
       expect(stack.flush_and_pop).to be nil
     end
 

spec/rackstash/logger_spec.rb

@@ -597,6 +597,10 @@ describe Rackstash::Logger do
         logger.add 0, 'I feel great'
       end
     end
+
+    it 'returns the yielded value' do
+      expect(logger.with_buffer { :hello }).to eql :hello
+    end
   end
 
   context 'with multiple threads' do