Fix standalone compilation error with std::string_view

fixes #1913

test/beast/http/field_compiles.cpp

@@ -0,0 +1,11 @@
+//
+// Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/boostorg/beast
+//
+
+// Test that header file is self-contained.
+#include <boost/beast/http/field.hpp>