`init_with` and `init_from_db` are almost the same code except decode
`coder`.

And also, named `init_from_db` is a little misreading, a raw values hash
from the database is already converted to an attributes object by
`attributes_builder.build_from_database`, so passed `attributes` in that
method is just an attributes object.

I renamed that method to `init_with_attributes` since the method is
shared with `init_with` to initialize an empty model object.