Bug #9438: Be less restrictive about the input to Horde_Secret::read/write()

Bug #9438: Be less restrictive about the input to Horde_Secret::read/write()

I originally assumed that it makes sense to pull the is_string() check
from Crypt_Blowfish into Horde_Secret (as Crypt_Blowfish would die on
anything not a string). I overlooked however that we had a strlen()
check before delegating to Crypt_Blowfish. As strlen(null) or
strlen(false) is int(0) both can be passed into
Horde_Secret::read/write() without causing problems.

Type casting the input into a string instead of checking the type
should fix the problems.