createdAt timestamp overwritten during findAndModify update w/upsert

[yangchristian]

For findAndModify with upsert, we run the update object against parseInsertObj since we might be inserting a new document. However, we may also just be doing an update, and the current implementation will always replace the createdAt and updatedAt fields, meaning createdAt gets overwritten. Will add a small fix not to overwrite createdAt.

We also have the scenario where we replace whatever updatedAt field we were passed. However, I believe we can count this situation as intended behavior. Thoughts @dimensia?