Add contents summary to CSV::Table#inspect

[ericgpks]

I thought that it is useful for us if this library has a function to return some rows results as a table. So changed the method to return a table with a header and rows.

[esparta]

I don't think is a good idea to change the signature and the return value of Table#inspect just to have a peek of data. If the user need that it may be a better idea to use Table#values_at(<range>), tho.

With this change there's a high risk some old scripts suddenly start to fail, some others (probably less) that was parsing or grep-ing the result will also fail.

[kou]

Thanks!