Add static_buffer (API Change):

* static_buffer is added

Note this is the same name from two versions ago, when
static_buffer was renamed to flat_static_buffer for
consistency and to clear the name for a circular static
buffer.

Actions Required:

* Callers who depend on static_buffer returning sequences of
  exactly length one should switch to flat_static_buffer.

include/beast/http/impl/chunk_encode.ipp

@@ -265,7 +265,7 @@ public:
     pointer
     operator->()
     {
-        return &(*this);
+        return &(**this);
     }
 
     const_iterator&