feat: not set is_unique by default

[weakish]

This is for compatibility.

[codecov-io]

Codecov Report

Merging #434 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
+ Coverage   86.83%   86.83%   +<.01%     
==========================================
  Files          29       29              
  Lines        2855     2856       +1     
==========================================
+ Hits         2479     2480       +1     
  Misses        376      376

Impacted Files	Coverage Δ
leancloud/conversation.py	83.05% <100%> (+0.29%)	⬆️

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