Resolved an issue with AWS::IAM::VirtualMFADevice#enable.

Passing in string values for a code like '099999' were raising
runtime errors from sprintf.  This change prevents string values
from getting passed through sprintf.  Added a test to cover this
behavior.

Fixes #288
Merge pull request #291 from aws/iam-fix

Resolved an issue with AWS::IAM::VirtualMFADevice#enable.