[Table] Error when using ng-content

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
`md-table` should wait until the `ng-content` replaced.

#### What is the current behavior?
`md-table` throws ` Cannot read property 'getColumnsDiff' of undefined`.

#### What are the steps to reproduce?
Put `ng-content` inside `md-table`.

https://plnkr.co/edit/RpOncq9lbwPcjAGjXSyR?p=preview

#### What is the use-case or motivation for changing an existing behavior?
Anyone that want to create a component with configurable `md-table` inside cannot do it.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular: `4.4.0-RC.0` in the plunker, but this is not working with any version.
Material: `2.0.0-beta.10`

首先，在链接的 Plunker 中重现 ng-content 位于 md-table 内部时的故障，然后跟踪 md-table 相对于投影内容替换访问 getColumnsDiff 的时机。当 md-table 等待投影内容，并且在可配置组件内部使用时不再抛出 undefined-property 错误时，即表示完成。

由索引模型根据 Issue 内容生成。