User agent feature tests

[doublnt]

You need to complete

• [x] unit tests and/or feature tests
• [x] documentation is changed or added

[doublnt]

• 修复了用户添加自定义的useragent 信息报 Exist key error
• 集成到了 Request 和 Client 中
• 添加了Client 的useragent 集成测试

[codecov-io]

Codecov Report

Merging #131 into master will decrease coverage by 0.25%. The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master     #131      +/-   ##
==========================================
- Coverage   97.22%   96.96%   -0.26%     
==========================================
  Files          53       53              
  Lines        2555     2573      +18     
  Branches      236      237       +1     
==========================================
+ Hits         2484     2495      +11     
- Misses         43       50       +7     
  Partials       28       28

Impacted Files	Coverage Δ
aliyun-net-sdk-core/DefaultAcsClient.cs	82.67% <100%> (+0.71%)	:arrow_up:
aliyun-net-sdk-core/AcsRequest.cs	91.02% <30%> (-8.98%)	:arrow_down:
aliyun-net-sdk-core/Http/UserAgent.cs	98.21% <95%> (ø)	:arrow_up:

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