Assign to this object the value of the specified rhs object, and return a reference providing modifiable access to this object. The contents of rhs are moved (in constant time) to this JsonObject if allocator() == rhs.allocator(); otherwise, all elements in this container are either destroyed or move-assigned to, and each additional element in rhs, if any, is move-inserted into this JsonObject. rhs is left in a valid but unspecified state.