Refactor treatment of status code and obsolete reason (API Change):

fix #398

A new enum status is added for the status code.

The function obsolete_reason returns default reason phrasing.

If a response has an empty reason, the serializer will
automatically insert the default reason phrase for the
status code.

test/http/message.cpp

@@ -296,13 +296,6 @@ public:
         }();
     }
 
-    void
-    testReasonString()
-    {
-        for(int i = 1; i <= 999; ++i)
-            BEAST_EXPECT(! reason_string(i).empty());
-    }
-
     void
     run() override
     {
@@ -312,7 +305,6 @@ public:
         testPrepare();
         testSwap();
         testSpecialMembers();
-        testReasonString();
     }
 };