admin: .set command can't handle spaces

[dgw]

I think I once discovered this a long, long time ago and forgot to document it. I vaguely remember hitting this problem way back when I first started using Sopel (at the time, Willie)… But here we are. I hit it again, and this time it's getting an issue so it can be fixed!

When PMing Sopel .set section.key spam eggs sausage and spam, the value in section.key of the config will become simply spam. Because admin.py uses trigger.group(4) as the value, it ignores anything after the first whitespace.

The .set command probably isn't used a whole lot, but it should still work as expected. As-is, it's useless for updating things like core.default_time_format, which almost always contain spaces.