[commands] Ensure that Context.command is the command in Command.can_run
[commands] Ensure that Context.command is the command in Command.can_run

Previously, Context.command was not guaranteed to be the actual command
being checked if it can run. This could be troublesome when
implementing help commands or when using the default help command.

This new change allows at least for the guarantee that Context.command
to be technically correct in Command.can_run.

Harmon758/discord.py,khazhyk/discord.py,Rapptz/discord.py,mgardne8/discord.py,imayhaveborkedit/discord.py,Harmon758/discord.py,rapptz/discord.py