A small fix to keep response body untouch for Joi validation.
Also make clear in Readme that custom properties don't make sense in case of metadata in headers.
Coverage remained the same at 100.0% when pulling cd880f5e88e12fc2f2a9c4322709cedd79d63178 on peveuve:meta-in-headers into e2399e1f12af45a7ca2f514998ddce2b63619f37 on fknop:master.
A small fix to keep response body untouch for Joi validation. Also make clear in Readme that custom properties don't make sense in case of metadata in headers.