fix:Add required check for customed type binding

[iksars]

What type of PR is this?

Check the PR title.

• [ ] This PR title match the format: <type>(optional scope): <description>
• [ ] The description of this PR title is user-oriented and clear enough for others to understand.
• [ ] Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

(Optional) Translate the PR title into Chinese.

为自定义类型添加required检查

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:Required tag assocated with customed type is invalid in previous version. This pr solves this problem by adding required check for customed type binding. What's more, I find that custom type should not only use json tag, because it's useless. This should be pointed out in the document. zh(optional):与自定义类型关联的必需标记在早期版本中无效。此pr通过为自定义类型绑定添加必需的检查来解决此问题。更重要的是，我发现自定义类型不应该只使用json标记，因为它是无用的。这一点应在文档中指出。

(Optional) Which issue(s) this PR fixes:

Fixes #1007

[CLAassistant]

All committers have signed the CLA.

[FGYFFFF]

感谢 pr，确实应该做一次 required 校验。请先签一下 CLA

[iksars]

感谢 pr，确实应该做一次 required 校验。请先签一下 CLA

CLA应该已经签了，我看测试golint没过，是不是要规范一下代码

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.54%. Comparing base (ad3f359) to head (253b97a). Report is 123 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1058   +/-   ##
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  Coverage    82.54%   82.54%           
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  Files           98       98           
  Lines        10025    10026    +1     
========================================
+ Hits          8275     8276    +1     
  Misses        1254     1254           
  Partials       496      496           

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