Remove exception_log from MetaInspector::Document#defaults

This was causing a bug where :warn_level options was not being properly
honored. Given that we already instantiate an ExceptionLog, just do so
as the default.

This is to resolve #94.
I really don't like diving into private instance variables like this, but I was struggling for a better way to test this.

Actually, I think I could just make a request that returns an exception and be sure it doesn't raise an exception. I can also then test #ok? and #exceptions to make sure they're populated properly.