API doc types and implementation for Bloom filter methods are inconsistent

[holgerd77]

The bloom filter tests recently added with https://github.com/ethereumjs/ethereumjs-vm/pull/330/files discovered that types of API docs (expecting Buffer) and implementation (taking String) for most methods (e.g. here) are not consistent.

There should be another look on this and be determined if taking Buffer values should be enforced or if the API docs should be changed.