look for pre/post-processors using _SINGLE and _MANY suffixes

This code previously was looking for pre/post-processors on `PUT`, and
applying them to `PATCH`, but the correct keys are `PUT_MANY`, `PUT_SINGLE`,
`PATCH_MANY` and `PATCH_SINGLE`.
Merge pull request #142 from glyphobet/put-processors-apply-to-patch

look for pre/post-processors using _SINGLE and _MANY suffixes