优化Properties#stringPropertyNames处理，jdk9以下版本此方法cpu消耗相对过高.

[wuwen5]

参考相关修改, http://hg.openjdk.java.net/jdk9/jdk9/jdk/rev/01a8615439f0#l2.164

[codecov-io]

Codecov Report

Merging #1072 into master will increase coverage by 17.43%.
The diff coverage is 88.88%.

@@              Coverage Diff              @@
##             master    #1072       +/-   ##
=============================================
+ Coverage     31.21%   48.65%   +17.43%     
- Complexity      111     1680     +1569     
=============================================
  Files            39      365      +326     
  Lines           977    10174     +9197     
  Branches        122     1030      +908     
=============================================
+ Hits            305     4950     +4645     
- Misses          628     4860     +4232     
- Partials         44      364      +320

Impacted Files	Coverage Δ	Complexity Δ
...trip/framework/apollo/internals/DefaultConfig.java	79.12% <88.88%> (ø)	26 <2> (?)
...ctrip/framework/apollo/portal/entity/bo/Email.java	0% <0%> (ø)	0% <0%> (?)
...ollo/portal/controller/ServerConfigController.java	8.33% <0%> (ø)	1% <0%> (?)
...ring/boot/ApolloApplicationContextInitializer.java	91.3% <0%> (ø)	5% <0%> (?)
...mework/apollo/biz/service/BizDBPropertySource.java	82.35% <0%> (ø)	12% <0%> (?)
...o/portal/spi/defaultimpl/DefaultLogoutHandler.java	16.66% <0%> (ø)	1% <0%> (?)
...amework/apollo/internals/ConfigServiceLocator.java	85.91% <0%> (ø)	12% <0%> (?)
...rip/framework/apollo/biz/service/AdminService.java	100% <0%> (ø)	2% <0%> (?)
...k/apollo/portal/controller/UserInfoController.java	8.33% <0%> (ø)	1% <0%> (?)
...ework/apollo/spring/util/BeanRegistrationUtil.java	72.72% <0%> (ø)	3% <0%> (?)
... and 317 more

---

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[coveralls]

Coverage increased (+0.008%) to 52.221% when pulling 962e4ef8c0576ba40f83a1b1feeea832299ac4d8 on wuwen5:properties#preform into dc65300 on ctripcorp:master.

[nobodyiam]

这里似乎是IDE自动更新为*了，建议只import实际需要的package

[nobodyiam]

这段代码虽然是从jdk9复制过来的，还是建议加一段UT来测试，以防后续被其它人改错逻辑。

[wuwen5]

ok已增加.

[nobodyiam]

感谢！

ut的assertEquals应该可以利用现有的stringPropertyNames实现来比较。

另外，是否能再增加一个properties为空的test case？

[nobodyiam]

assertEquals(someProperties.stringPropertyNames(), propertyNames) ?

[wuwen5]

已增加.

[nobodyiam]

Thanks!