Request data can be read only once

[knazarov]

If I call req:param() then req:json() will fail subsequently, because it won't be able to read the request body using req:read(), because req:read() is not reentrant.

Request body should be cached somewhere. At least if content-type is application/json

[bigbes]

:read_cached() is already implemented

[iDneprov]

:read_cached() is already implemented

Please add :read_cached() to readme.