Set preserve to true by default

[jmatsushita]

It is probably a fair expectation that data isn't deleted by default in unpredictable ways so I'd suggest we set the preserve option to true as a breaking change and allow to get the old behavior back by explicitly setting preserve to false.

This will also help with the simplicity of the conflict/overwrite API at #46

cc @BigBlueHat

[mcollina]

Go ahead and do what you think is right. I will have little time to dedicate to this in the coming weeks.

[jmatsushita]

Ok. Last question, should we bump major version again or consider that this is part of 1.x breaking changes (given I don't think a huge amount of people depend on it)?

[mcollina]

I'd go ahead and push it as 1.0.1.