Improve edge case handling

[h0tl33t]

This PR covers the following:

• Clean up Sprig::Reap::Association by using some of the built-in methods on ActiveRecord::Reflection::AssociationReflection
• Handle belongs_to associations that can be nil (would previously throw an error)
• Deal with model attributes that are wrapped by Ruby objects by using read_attribute instead of send (should fix #5)

[ltk]

There's an undocumented sprig_file feature in Sprig, that lets you store files within a files directory and upload them to records just like they were uploaded from a browser. For example, if you had a cat picture at db/seeds/development/files/cat.png, you could attach it to a record with <%= sprig_file('cat.png') %>.

You might consider handling files differently than other attributes, by first adding them to the files dir, then including a sprig_file reference in the seed file.

(I shall work on documenting this feature in Sprig.)

[efatsi]

:+1: to these updates

[ltk]

looks good other than that filename uniqueness stuff :+1:

[h0tl33t]

Thanks guys!