Add span_body

include/beast/http.hpp

@@ -23,6 +23,7 @@
 #include <beast/http/read.hpp>
 #include <beast/http/rfc7230.hpp>
 #include <beast/http/serializer.hpp>
+#include <beast/http/span_body.hpp>
 #include <beast/http/status.hpp>
 #include <beast/http/string_body.hpp>
 #include <beast/http/string_view_body.hpp>