seq? -> sequential? to be able to handle vectors in pom/relativize

The current implementation would break if a middleware plugin emitted a vector for any of the keys in L20 .

It wraps the output in a vec, so it's reasonable to assume it could take a vec as well.
Merge pull request #1649 from MarcoPolo/sequential-fix-in-relativize

seq? -> sequential? to be able to handle vectors in pom/relativize