#26)

Unmarshal JWKs with unsupported key type or algorithm into empty
JSONWebKeys. Ignore invalid JWKs when unmarshalling JWK Sets.

This reverts commit 3e2bbef, "Unmarshal
jwk keys with unsupported key type or algorithm into empty ... (#26)"

I accidentally merged that PR into v3, but per our README, the v3 branch
is getting security updates but not functionality updates.

Backport #167 

git cherry-pick 99b346c

* cipher: fix panic on KeyUnwrap of too-short slice

* jwe: don't call KeyUnwrap on empty (encrypted) key

Also don't call `aead.decrypt` on an empty key.

* test: make asymmetric_test more precise

These two test cases were passing a nil recipient, and checking for "any error"
instead of a specific error, which meant that introducing a nil recipient check
in `decryptKey` caused the test to stop testing what it meant to test, but
continue passing. Now we check for a specific error.

* test: TestKeyUnwrapShort

* jwe: TestEmptyEncryptedKey

* test: add `shorten` and `empty` corruptors

* fix import path

Targeting the v3 branch (backport).