[bug 581] Fix parameter checking if no data-type

Fix bug #581, which reported that the deployment of the simplecharm
failed because the filename parameter was invalid.

This turned out to be an issue introduced with the parameter
type-checking added for R5. This has been fixed, and an integration test
has been added that exercises the simplecharm example.

Signed-off-by: Adam Israel <adam.israel@canonical.com>