Changed a few DBI methods to dispatch over the first two arguments

[bborgesr]

(as per the default definition in DBI)

@krlmlr, thanks for letting me know about this issue. Are the changes here enough to close #55?
(To be on the safe side, I also went ahead and made the same change to other functions in this file that also have a default implementation in DBI with double dispatch.)

[krlmlr]

Thanks. To test this, can you please run R CMD check with the dev version of DBI? When do you plan to release to CRAN?

[bborgesr]

@krlmlr, I don't get any errors/warnings/messages on devtools::check() for pool, no matter which branch I'm in (master or dispatch), even if I'm on the dev version of DBI... So, it seems like you are catching something I can't replicate.

I released pool somewhat recently and I didn't have any plans for another release soon. But if this is a breaking change, I can do another release, np. When are you planning on releasing DBI?

[krlmlr]

Thanks for looking into it! DBI 0.8 is on CRAN, I mentioned that I contacted you regarding the check problems in pool.

[bborgesr]

Thanks @krlmlr, I also submitted pool 0.1.4 to CRAN yesterday and am now waiting for it to be accepted.