nearestKNeighbors does not handle seqlevel pruning

[yeyuan98]

In current version (commit https://github.com/Bioconductor/GenomicRanges/commit/d20afa47c9e7acbf213bbdc1ded043bd817f2458) nearestKNeighbors will fail if subject contains seqlevels not used by x, due to this line in .nearestKNeighbors:

seqlevels(subject) <- seqlevels(x)

Changing that line to the following should suffice presumably:

seqlevels(subject, pruning.mode = "tidy") <- seqlevels(x)

Would be more than happy to create a pull request or contribute in any way as needed. Thanks a lot for the great Bioconductor infrastructure packages.