Add API that will allow/deny/warn automatic decoding of json based on content-type.

[socketpair]

Now, aiohttp issue warning if await response.json() called and response does not contain correct content-type.
This should be configurable. I suggest to to require content_type.startswith('application/json'). Aiohttp's users must be able to turn this check off and also specify his own MIME-type that should pass this check. I understand, that many broken HTTP-servers give JSON-response without setting correct content-type. I think, we should fight against these servers.

[fafhrd91]

makes sense

[fafhrd91]

added

[pnuckowski]

What about application/vnd.api+json is it correct content-type?

[socketpair]

@pnuckowski it is not standard (i.e. RFC recommended) content-type.

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