Tidy up HTTP reason_string (API Change):

fix #182

reason_string() is now located in <beast/http/message.hpp>
and returns a string view rather than char const*.

include/beast/http/impl/message.ipp

@@ -9,6 +9,7 @@
 #define BEAST_HTTP_IMPL_MESSAGE_IPP
 
 #include <beast/core/error.hpp>
+#include <beast/core/string_view.hpp>
 #include <beast/http/concepts.hpp>
 #include <beast/http/rfc7230.hpp>
 #include <beast/core/detail/ci_char_traits.hpp>
@@ -257,6 +258,71 @@ prepare(message<isRequest, Body, Fields>& msg,
             "invalid version for Connection: upgrade", __FILE__, __LINE__);
 }
 
+namespace detail {
+
+template<class = void>
+string_view
+reason_string(int status)
+{
+    switch(status)
+    {
+    case 100: return "Continue";
+    case 101: return "Switching Protocols";
+    case 200: return "OK";
+    case 201: return "Created";
+    case 202: return "Accepted";
+    case 203: return "Non-Authoritative Information";
+    case 204: return "No Content";
+    case 205: return "Reset Content";
+    case 206: return "Partial Content";
+    case 300: return "Multiple Choices";
+    case 301: return "Moved Permanently";
+    case 302: return "Found";
+    case 303: return "See Other";
+    case 304: return "Not Modified";
+    case 305: return "Use Proxy";
+    case 307: return "Temporary Redirect";
+    case 400: return "Bad Request";
+    case 401: return "Unauthorized";
+    case 402: return "Payment Required";
+    case 403: return "Forbidden";
+    case 404: return "Not Found";
+    case 405: return "Method Not Allowed";
+    case 406: return "Not Acceptable";
+    case 407: return "Proxy Authentication Required";
+    case 408: return "Request Timeout";
+    case 409: return "Conflict";
+    case 410: return "Gone";
+    case 411: return "Length Required";
+    case 412: return "Precondition Failed";
+    case 413: return "Request Entity Too Large";
+    case 414: return "Request-URI Too Long";
+    case 415: return "Unsupported Media Type";
+    case 416: return "Requested Range Not Satisfiable";
+    case 417: return "Expectation Failed";
+    case 500: return "Internal Server Error";
+    case 501: return "Not Implemented";
+    case 502: return "Bad Gateway";
+    case 503: return "Service Unavailable";
+    case 504: return "Gateway Timeout";
+    case 505: return "HTTP Version Not Supported";
+
+    case 306: return "<reserved>";
+    default:
+        break;
+    }
+    return "<unknown-status>";
+}
+
+} // detail
+
+inline
+string_view const
+reason_string(int status)
+{
+    return detail::reason_string(status);
+}
+
 } // http
 } // beast