Add BufferStack#pop to remove the top-most Buffer without flushing it

This can be used for certain advanced constructs where the Buffer needs
to be manually flushed later, after it was already removed from the
stack.

lib/rackstash/buffer_stack.rb

@@ -49,13 +49,18 @@ module Rackstash
       buffer
     end
 
-    # Remove the top-most Buffer from the internal stack.
+    # Remove the top-most {Buffer} from the internal stack without flushing it.
     #
-    # If there was a buffer on the stack and it has pending data, it is flushed
-    # to the {#sink} before it is returned.
+    # @return [Buffer, nil] the poped {Buffer} or `nil` if there was no {Buffer}
+    #   to remove.
+    def pop
+      @stack.pop
+    end
+
+    # Flush and remove the top-most {Buffer} from the internal stack.
     #
-    # @return [Buffer, nil] the removed {Buffer} or `nil` if there was no
-    #   {Buffer} to remove
+    # @return [Buffer, nil] the poped and flushed {Buffer} or `nil` if there was
+    #   no {Buffer} to remove
     def flush_and_pop
       buffer = @stack.pop
       buffer.flush if buffer

spec/rackstash/buffer_stack_spec.rb

@@ -85,4 +85,30 @@ describe Rackstash::BufferStack do
       stack.flush_and_pop # no further buffer, thus `#flush` is not called again
     end
   end
+
+  describe '#pop' do
+    it 'removes a buffer from the stack' do
+      stack.push
+      expect { stack.pop }
+        .to change { stack.instance_variable_get(:@stack).count }.from(1).to(0)
+    end
+
+    it 'does nothing if there is no buffer' do
+      expect(stack.instance_variable_get(:@stack).count).to eql 0
+      expect(stack.pop).to be_nil
+    end
+
+    it 'returns the pop\'ed Buffer' do
+      stack.push
+      expect(stack.pop).to be_a Rackstash::Buffer
+      expect(stack.pop).to be nil
+    end
+
+    it 'does not flushes the buffer' do
+      new_buffer = stack.push
+
+      expect(new_buffer).not_to receive(:flush)
+      stack.pop
+    end
+  end
 end