SSL.File: Unify code paths in write().

Reduce the amount of duplicated code in write() by unifying the code
for the array and string cases.

This has the added benefit of having the array case actually being tested.
Runtime: Increased strictness of variant dispatcher.

The variant dispatcher regarded all objects (even those without `()())
as callable. This was not a good idea.

Fixes some of the issues with the Crypto.RSA testsuite.