fix nil attachment on stream in custom encoder on sorted map

[vano144]

Hello, thank you for a library !

This pr fixes next problem:

1. We create jsoniter.Stream with SortMapKeys enable
2. Pass attachment to created stream
3. Try to serialize map with custom types
4. Attachment will be null on stream in Encode method

Example in test - Test_custom_encoder_attachment

[codecov]

Codecov Report

Merging #423 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #423      +/-   ##
==========================================
+ Coverage   86.46%   86.46%   +<.01%     
==========================================
  Files          41       41              
  Lines        5105     5107       +2     
==========================================
+ Hits         4414     4416       +2     
  Misses        555      555              
  Partials      136      136

Impacted Files	Coverage Δ
reflect_struct_encoder.go	89.07% <100%> (+0.09%)	⬆️
reflect_map.go	87.74% <100%> (+0.06%)	⬆️
reflect_struct_decoder.go	81.35% <0%> (ø)	⬆️

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