[commands] Fix when_mentioned when handling nicknames.
[commands] Fix when_mentioned when handling nicknames.

jhgg/discord.py,rapptz/discord.py,Rapptz/discord.py,imayhaveborkedit/discord.py,lgkern/PriestPy,Harmon758/discord.py,Theoretical/UselessBot,mgardne8/discord.py,Harmon758/discord.py,variablehair/Eggplantato,khazhyk/discord.py