Improve condition for whether force_encoding is needed

* oracle/truffleruby#3506 was fixed,
  instead of hardcoding Ruby engines and versions, it seems better
  to do the check whether the force_encoding is needed.

Author: eregon

lib/redis_client/ruby_connection/buffered_io.rb

@@ -147,9 +147,6 @@ def ensure_remaining(bytes)
         end
       end
 
-      RESET_BUFFER_ENCODING = RUBY_ENGINE == "truffleruby"
-      private_constant :RESET_BUFFER_ENCODING
-
       def fill_buffer(strict, size = @chunk_size)
         remaining = size
         start = @offset - @buffer.bytesize
@@ -174,7 +171,7 @@ def fill_buffer(strict, size = @chunk_size)
             if empty_buffer
               @offset = start
               empty_buffer = false
-              @buffer.force_encoding(Encoding::UTF_8) if RESET_BUFFER_ENCODING
+              @buffer.force_encoding(Encoding::UTF_8) unless @buffer.encoding == Encoding::UTF_8
             else
               @buffer << bytes.force_encoding(Encoding::UTF_8)
             end