Deprecate string_param (API Change):

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has been replaced by `string_view`.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

example/doc/http_examples.hpp

@@ -316,7 +316,7 @@ void do_server_head(
         // set of headers that would be sent for a GET request,
         // including the Content-Length, except for the body.
         res.result(status::ok);
-        res.set(field::content_length, payload.size());
+        res.content_length(payload.size());
 
         // For GET requests, we include the body
         if(req.method() == verb::get)