修复model卸载后无法重新挂载bug(dynamic)

[lf7817]

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• 修改注册model条件
  dynamic.js

var cached = {};

function registerModel(app, model) {
  model = model.default || model;

  if (!cached[model.namespace]) {
    app.model(model);
    cached[model.namespace] = 1;
  }
}

当执行app.unmodel卸载model时，并没有修改cached缓存，所以重复挂在model时，永远不会执行app.model, 应该从app._models中判断

function registerModel(app, model) {
  model = model.default || model;
  if (
    !app._models.some(item => {
      return item.namespace === model.namespace;
    })
  ) {
    app.model(model);
  }
}

• close app.unmodel 之后， dynamic 不会重复注册model #1482

[coveralls]

Coverage decreased (-0.03%) to 87.25% when pulling 9659378 on lf7817:master into 99ec56c on dvajs:master.

[sorrycc]

补个用例吧。

[stale]

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