Fixed ValueError when setting enum properties

[vkribo]

In setter of an enum property there must be a cast of value to int.

Code:
conn = krpc.connect()
mj = conn.mech_jeb
smart_ass = mj.smart_ass
smart_ass.autopilot_mode = mj.SmartASSAutopilotMode.surface

Assigning to the autopilot_mode property was causing ValueError.
The same thing happens with other enum properties.

[Genhis]

Looks good :-)
Can you add a changelog entry with your pull request link in brackets?

Something like

### Fixed
- <text here> ([#4](https://github.com/Genhis/KRPC.MechJeb/pull/4))