ENH: use union of bad channels when merging Evoked and Cov

Until now, when merging Evoked, the bad channels of the first Evoked instance was used while for Cov merging was only supported when all Cov instances have the same bad channels. I think it is better to instead use the union of bad channels, i.e., if a channel is bad in one instance it will also be bad in the merged instance.