make io.Reader compressible in responses

[gudvinr]

As you can pipe data from reader to write through io.Copy it is possible to compress any reader quite easily.

And, since []byte and string can be converted to io.Reader too the whole implementation looks tidier. At least in my eyes.

Chechking for Size methods is done because bytes.Reader and strings.Reader do provide these. If io.Reader doesn't have Size method and encoding allows that it'd be compressed unconditionally.

406 Not Acceptable will be returned as per MDN. Only if uncompressed encoding explicitly disabled (e.g. identity;q=0) and we don't support any compression that client wants. That should be quite rare occasion I suppose.

This should be merged after #4370.

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