InstanceResource.load fails to parse rfc2822 dates for unicode strings

The InstanceResource.load method only parses dates correctly for python 3 environments. For python 2, they remain in the rfc2822 string format.

```
if key.startswith("date_") and isinstance(entries[key], str):
```

should be 

```
if key.startswith("date_") and isinstance(entries[key], six.string_types):
```
