Forbid duplicating of web.Application and web.Request

[asvetlov]

I've found both Application and Request has .copy() method with unpredictable behavior.
We should forbid this one.

There are two options:

1. Don't inherit web.Application and web.Request from dict but implement MutableMapping abc from scratch.
2. Override .copy with raising NotImplementedErrorr exception.

I'm inclining to second choice.

The patch is trivial but requires two additional tests in our suite and docs change.

Any volunteers?

[jashandeep-sohi]

I'll make a PR with option 2.

[asvetlov]

Thanks

[lock]

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