Better coherence with Rails param scrubbing behavior

Also eliminates the potential DOS vulnerability of calling `to_sym` on all params (symbols aren't garbage collected).
Handle arrays which may contain hashes when param scrubbing
Only generate the sensitive params regexp once
Revert "Only generate the sensitive params regexp once"

This reverts commit 427691b2f9d4002b95fdf1a3a756ddb0847ed02d.
memoize sensitive params regexp
Merge branch 'pmen-master'