Handle invalid deflate frames:

This fixes a problem where an assert is generated or an
error is ignored when an invalid deflate stream is produced
after appending the final empty deflate block.

include/boost/beast/websocket/impl/error.ipp

@@ -43,6 +43,7 @@ public:
         case error::closed: return "WebSocket connection closed normally";
         case error::handshake_failed: return "WebSocket upgrade handshake failed";
         case error::buffer_overflow: return "WebSocket dynamic buffer overflow";
+        case error::partial_deflate_block: return "WebSocket partial deflate block";
         }
     }