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has been replaced by `string_view`.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/http/server/small/http_server_small.cpp

@@ -173,7 +173,7 @@ private:
     {
         auto self = shared_from_this();
 
-        response_.set(http::field::content_length, response_.body().size());
+        response_.content_length(response_.body().size());
 
         http::async_write(
             socket_,