Fixed rdoc -E

rdoc -E depended on the original file existing.  In the case it didn't
it would report the file as unparseable.  Original patch by Nobuyoshi
Nakada as ruby commit r35532.

The patch was updated to handle modeline support and rearrange the tests
to work properly.  Now can_parse will return a parser which will get
checked later by RDoc::Parser::for.

Ruby bug #6392
Ensure --extension is recorded in the parser list

Follow up commit to the rdoc -E fix