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*.

test/http/message.cpp

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