Add streambuf write():

This function performs serialization of a variable list of arguments
to a streambuf. It accepts a wide variety of argument types, using
boost::asio::buffer and boost::lexical_cast where possible.

test/write_streambuf.cpp

@@ -0,0 +1,35 @@
+//
+// Copyright (c) 2013-2016 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)
+//
+
+// Test that header file is self-contained.
+#include <beast/write_streambuf.hpp>
+#include <beast/streambuf.hpp>
+#include <beast/detail/unit_test/suite.hpp>
+
+namespace beast {
+
+class write_streambuf_test : public beast::detail::unit_test::suite
+{
+public:
+    void run() override
+    {
+        streambuf sb;
+        std::string s;
+        write(sb, boost::asio::const_buffer{"", 0});
+        write(sb, boost::asio::mutable_buffer{nullptr, 0});
+        write(sb, boost::asio::null_buffers{});
+        write(sb, boost::asio::const_buffers_1{"", 0});
+        write(sb, boost::asio::mutable_buffers_1{nullptr, 0});
+        write(sb, s);
+        write(sb, 23);
+        pass();
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(write_streambuf,core,beast);
+
+} // beast