add support for RSA 1.5 transport of AES keys

[toshok]

https://www.w3.org/TR/xmlenc-core/#sec-RSA-1_5

Slight departure from the spec, in that the spec reads:

Implementations must support this key transport algorithm for transporting 192-bit TRIPLEDES keys. Support of this algorithm for transporting other keys is optional.

This PR only implements AES key transport, not tripledes.

This version of the spec also has this passage:

Note: Implementation of RSA v1.5 is not recommended due to security risks associated with the algorithm.

But I want to support assertions from IDPs that aren't exactly up-to-date on the current xmlenc spec.

Fixes things for me, not sure about testing this, though.