reject 1xx responses with a body in prepare_payload

[sahvx655-wq]

prepare_payload on a response is meant to reject a body for the status codes RFC 7230 says cannot carry one, but the 1xx branch never fires. At message.hpp:393 status_class(this->result()) is a functional-style cast rather than a call to to_status_class, so it reinterprets the numeric status (e.g. 100) as a status_class value and compares it against status_class::informational, whose underlying value is 1. The test is false for every real 1xx code, so a 100/101/102/103 response built with a non-empty body sails through and even gets a synthesised Content-Length. 204 and 304 still throw because they are checked explicitly by value; only the whole 1xx class was broken.

The sibling need_eof a few lines up at message.hpp:345 does the same classification correctly with to_status_class, which is what tipped me off. Switching line 393 to to_status_class makes the 1xx case throw invalid_argument like 204/304, so the library no longer emits a 1xx response with a body and a Content-Length that RFC 7230 3.3.2 forbids and that an intermediary can read as message framing. Valid responses are untouched; the case added to testPreparePayload fails on the four 1xx codes before the change and passes after.