Fix TypeError when constructing a channel in start_private_message.
Fix TypeError when constructing a channel in start_private_message.

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