WIP: don't set created_at to now() by default for updates

[KMPolakowski]

fixes issue #110

[Fs02]

hmmm, the test fails because the result returned by db adapter contains location information, even though it's a zero time.
It's a valid result, but just hard to do the exact test. (fixable by only setting this value if not zero:

rel/convert.go

Line 54 in 3fb088c

*d = s

)

Edit: just noticed there's different problem for MySQL, where they don't accept 0000-00-00 as date time.
Edit: seems we need to rethink this solution based on the issue.