Allow querying reserved keys

I was trying to use pykeepass to debug a KeePass database created by another tool. Precisely I wanted to check if the otp field wad protected or not. Yet if I attempt to do `myentry.is_custom_property_protected('otp')`, I would get this error: 
```
AssertionError: otp is a reserved key
```

Please allow access (at least read-only) to reserved key values & attributes

