feat: model文件名是默认的 dva model namespace

[xc1427]

比如在 model/ 下面建立 user.js 文件作为一个 dva model 的定义，
那么该 model 的 namespace 默认为 'user'，除非在 user.js 文件中显式地给出 namespace

[coveralls]

Pull Request Test Coverage Report for Build 546

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 21.69%

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Totals
Change from base Build 542:	0.0%
Covered Lines:	518
Relevant Lines:	2611

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💛 - Coveralls