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.
Test Document#(<url>, warn_level: <warn_level>) behavior

Update specs to test only behavior of public methods
Merge pull request #95 from jgandt/fix_document_warn_level_default

Remove exception_log from MetaInspector::Document#defaults