Allow multi update without $ operator

An update with multi: true is currently only allowed if it uses $ operators, such as $set to modify individual fields. If you try to replace the entire document, you get the error message "multi update only works with $ operators".

After some discussion, it was concluded that there are indeed cases where it makes sense to replace a number of documents with a given, single document (except the _id field, obviously).

So this ticket requests to remove the above error condition and carry out the operation as usual, i.e. replace all the selected documents with the update document.