small fix to test
rethrow exception if it is unrelated to conversion
fixes in checking for class
type checking for DataFrame and Factor

Make sure that the conversion for both DataFrame and Factor check their
respective types. This is so that the `R::DataConvert` exception
handling works properly and only fails if the specific type's conversion
handling is unimplemented.
make sure checking for blessed references parses correctly in all cases (e.g., a string doesn't get treated as a package)
reorder the plugins for conversion from Perl to R

Factor conversion must come first because a PDL::Factor is also a PDL.
Perl conversion must be last because it has a catch-all condition.
`op_equal_all` now uses eval with error checking