Stop splatting clean hash argument in Bufferstack#push

The optional keyword arguments are already cleaned by the keyword splat
in BufferStack#push. They are automatically unsplatted when calling
Buffer#initialize anyway.

lib/rackstash/buffer_stack.rb

@@ -47,8 +47,8 @@ module Rackstash
     # @param buffer_args [Hash<Symbol => Object>] optional arguments for the new
     #   {Buffer}. See {Buffer#initialize} for allowed values.
     # @return [Buffer] the newly created buffer
-    def push(**buffer_args)
-      buffer = Buffer.new(sink, **buffer_args)
+    def push(buffer_args = {})
+      buffer = Buffer.new(sink, buffer_args)
       @stack_mutex.synchronize do
         @stack.push buffer
       end

lib/rackstash/logger.rb

@@ -376,10 +376,10 @@ module Rackstash
     # @param buffer_args [Hash<Symbol => Object>] optional arguments for the new
     #   {Buffer}. See {Buffer#initialize} for allowed values.
     # @return [Object] the return value of the block
-    def with_buffer(**buffer_args)
+    def with_buffer(buffer_args = {})
       raise ArgumentError, 'block required' unless block_given?
 
-      buffer_stack.push(**buffer_args)
+      buffer_stack.push(buffer_args)
       begin
         yield
       ensure