Merge pull request Tencent#862 from Tencent/dev

for v1.2.13

Merge pull request Tencent#817 from Tencent/dev

upgrade Android & flutter deps

Merge pull request Tencent#779 from Tencent/dev

flutter unit test not ready

Merge pull request Tencent#710 from Tencent/dev

fix some javadoc warning

Update README.md

Merge pull request Tencent#680 from Tencent/dev

update flutter example podfile

Merge pull request Tencent#606 from Tencent/dev

prepare for flutter v1.2.8

Merge pull request Tencent#595 from Tencent/dev

support golang

Merge pull request Tencent#577 from Tencent/dev

update readme for flutter

Merge pull request Tencent#561 from Tencent/dev

for v.1.2.5