Fix merge modifying attr map

Author: malmeloo

src/circuit.mjs

@@ -170,6 +170,7 @@ export class HeadlessCircuit {
                 cellArgs.graph = this._makeGraph(subcircuits[dev.celltype], subcircuits, { nested: true });
             const cell = new cellType(cellArgs);
             const cellAttrs = _.merge(
+                {},
                 this._cellAttributes[cell.get('type')],
                 this._cellAttributes[cell.get('celltype')]
             );