Fix BigDecimal#to_s method to accept format argument

[giddie]

I broke things (Arel, in particular) with the implementation of #to_s I committed :( This fixes it all up.

[robwilliams]

How does it break Arel?

[robwilliams]

Can you rebase please? I merged your other pull request :s

[giddie]

Apologies for the mess: I'd hoped to sneak in the fix under the radar, but you were too quick for me :p

The issue with Arel is that when constructing SQL to store BigDecimals, it calls BigDecimal#to_s("F"). I had tweaked #to_s to accept a format argument without thinking to check what the format argument was. I had just assumed it would accept an sprintf-compatible format string, but in fact it's a special format code (hence "F"). As a result, my #to_s implementation, called from Arel, would convert every BigDecimal to the string "F", which the database simply rejects as badly-formatted SQL.

The new implementation uses the format string if supplied with #to_s, and if no parameter was supplied it falls back to an sprintf-based formatting.