Fix standalone compilation error with std::string_view

fixes #1913

test/beast/http/CMakeLists.txt

@@ -26,6 +26,7 @@ add_executable (tests-beast-http
     empty_body.cpp
     error.cpp
     field.cpp
+    field_compiles.cpp
     fields.cpp
     file_body.cpp
     message.cpp

test/beast/http/Jamfile

@@ -16,6 +16,7 @@ local SOURCES =
     dynamic_body.cpp
     error.cpp
     field.cpp
+    field_compiles.cpp
     fields.cpp
     file_body.cpp
     message.cpp

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>