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.
You don't need to do an `instance_variable_get` here, as this instance variable has a reader:

https://github.com/jaimeiniesta/metainspector/blob/master/lib/meta_inspector/exception_log.rb#L7