CompositeFileProvider should implement IDisposable

[aspnet-hello]

From @jiatao99 on Wednesday, December 13, 2017 8:20:12 AM

1. I implement my own IFileProvider (like Zip) and implement IDisposable
2. I add my provider into CompositeFileProvider
3. I add CompositeFileProvider into ServiceCollection

Result: my provider will not be disposed properly

CompositeFileProvider should implement IDisposable and dispose inner providers.

Copied from original issue: aspnet/FileSystem#307

[muratg]

We are closing this issue because no further action is planned for this issue. If you still have any issues or questions, please log a new issue with any additional details that you have.