Don't mix hash arguments with keyword arguments

Since Ruby 2.7 differences explicit keyword arguments from implicit hash
arguments, we should also avoid mixing them. By using explicit keyword
arguments, we avoid warning in Ruby 2.7 and errors in Ruby 3.0.

lib/rackstash/buffer_stack.rb

@@ -43,8 +43,8 @@ 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(@flows, buffer_args)
+    def push(**buffer_args)
+      buffer = Buffer.new(@flows, **buffer_args)
       @stack.push buffer
 
       buffer

lib/rackstash/logger.rb

@@ -401,8 +401,8 @@ module Rackstash
     # @param buffer_args [Hash<Symbol => Object>] optional arguments for the new
     #   {Buffer}. See {Buffer#initialize} for allowed values.
     # @return [Buffer] the newly pushed {Buffer} instance
-    def push_buffer(buffer_args = {})
-      buffer_stack.push(buffer_args)
+    def push_buffer(**buffer_args)
+      buffer_stack.push(**buffer_args)
     end
 
     # Remove a previously pushed {Buffer} from the {BufferStack} for the current
@@ -451,10 +451,10 @@ module Rackstash
     #   removed from the {BufferStack} again and is always flushed
     #   automatically.
     # @return [Object] the return value of the block
-    def capture(buffer_args = {})
+    def capture(**buffer_args)
       raise ArgumentError, 'block required' unless block_given?
 
-      buffer_stack.push(buffer_args)
+      buffer_stack.push(**buffer_args)
       begin
         yield
       ensure