Write failing test for coercion on initialization

[bobbytables]

I described part of the piece I was trying to do in #471 but I didn't explain well enough. This is really for coercion on initialization of Dash with coercion included, these cases show what I mean.

[bobbytables]

I'm seeing a test fail when I update the code to do what I was desiring, which tells me this is intended. So now I ask: Why?

Test that fails: https://github.com/intridea/hashie/blob/master/spec/hashie/extensions/coercion_spec.rb#L298

3) Hashie::Extensions::Coercion#coerce_key coercing core types does not coerce nil
     Failure/Error: expect(instance[:foo]).to_not eq('')

       expected: value != ""
            got: ""

       (compared using ==)
     # ./spec/hashie/extensions/coercion_spec.rb:323:in `block (4 levels) in <top (required)>'

This is weird to me, why not convert nil on core types when its explicit?

[dangerpr-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#472](https://github.com/intridea/hashie/pull/472): Write failing test for coercion on initialization - [@bobbytables](https://github.com/bobbytables).

Generated by 🚫 Danger

[bobbytables]

I made my tests pass, but like I said, it breaks another so this was intended in some way. So I'll leave this open to discuss why we're not converting core objects when they're nil. This was a pretty big curveball for me so I'd love to know why and maybe we can sand this part down and make it at least more clear in the README.

[dblock]

Good work. Lets see what @michaelherold has to say!

[bobbytables]

Ping, @michaelherold 😄

[michaelherold]

I'm just taking a look at this; sorry for the delay! In your original issue you're talking about a Trash but what you're dealing with here is a Dash with the Coercion mix-in added. Can you explain the mismatch there?

I ask because there's a difference between coercing a value and setting a default when missing. Your PR here is talking about coercing a value, which requires a value to coerce. Setting a default should happen when the value is missing.