Added correct cloning of ReadPreferences.

Fixes #30.
Problem was with cloning `options.readPreference` in `Query#_optionsForExec()`.
It was cloned to a simple object, which didn't work well with Mongo driver.
Not it is cloned to another ReadPreference object.

Test added.

Also removed unnecessary `{1}` from ObjectId checking regexp.
fixed; check for ObjectId constructor
release 0.5.2
whitespace