API Changes:
`string_param`, which was previously the argument type when setting field values
has been replaced by `string_view`. Because of this, it is no longer possible to
set message field values directly as integrals.
Users are required to convert numeric arguments to a string type prior to calling
`fields::set` et. al.
Beast provides the non-allocating `to_static_string()` function for this purpose.
To set Content-Length field manually, call `message::content_length`.
fixes#1956
Richard Hodges
14c3d50b57