Correct payment sandbox endpoint and add a method to get sandbox sign key (#666)
Correct payment sandbox endpoint and add a method to get sandbox sign key (#666)

* Correct payment sandbox endpoint and add a method to get sandbox sign key.

* Fixing style warnings.

* Internalize acquiring sandbox sign key process.

* Fixing styles.

* Remove one empty line.

* Depedency injection for cache backend.

* Fixed a typo.

* Extract method.

* rename.

* Add Cache import; remove try catch and use return instead of else.

* Reorder use statement.

* Remove one empty line.

* Add one empty line to meet style check.

* Re-add this line.

allen05ren/wechat,acgrid/wechat,tianyong90/wechat,tianyong90/wechat,overtrue/wechat,hareluya/wechat,overtrue/wechat,ac1982/wechat,acgrid/wechat,allen05ren/wechat,ac1982/wechat