What I expected to happen was...:

Running '/difficulty (difficulty)' and then restarting the server sometime after will remain on the chosen difficulty.

What actually happened was...:

After the server is restarted, the difficulty returns to what it was previously. This appears to be because /difficulty does not update the 'server.properties' file.

Steps to Reproduce:
1. Start the server with any difficulty. For example, easy.
2. Run /difficulty with no argument. This will return the current difficulty (easy in this case).
3. Run /difficulty with any other difficulty supplied. For example, normal.
4. Run /difficulty with no argument. This will return the newly selected difficulty (normal in this case).
5. Restart the server and then run /difficulty with no argument again. This will return the previous difficulty (easy in this case).