Don't modify creation date when editing bookmark via api

Hi.

When changing a bookmark with the API, the only way to do this is via `posts/add?replace=yes&...`.
When editing a bookmark the expected behaviour is, that the creation date is unchanged and the modification date will change.

Currently both dates are changed when using the api function. A workaround would be to send a date `dt=...`, but then the modification date is not accurate, it will be set to the same date as the new creation date.

With this pull request, when `replace=yes` is set, the bookmark will be "updated" and the `dt` option will be ignored. This is compatible to the delicious api.

I wrote a test for that, which passed in my setup.

I'm creating a new pull request for this (instead of using https://github.com/cweiske/SemanticScuttle/pull/9) because, this seems like a bug, that should be fixed in the current api.
I will fix it in the new api as well.