Fix JSON::Serializable on certain recursively defined types

[HertzDevil]

Fixes #13337. Does not affect #13238 (comment).

I couldn't reproduce the same error with YAML

[straight-shoota]

I think this removes all uses of value[:type]. So we can remove that from the preparation macro loop.

And even though the issue does not reproduce with YAML, I'd still like to add the same spec there to make sure of it.

[daliborfilus]

JSONAttrValue() wrapper is not required here for the issue to manifest. Why it was added? Doesn't that add possibility to hide the issue in the future (for some other bugs)?

[straight-shoota]

Good catch. The spec setup can actually be simplified a bit:

--- i/spec/std/json/serializable_spec.cr
+++ w/spec/std/json/serializable_spec.cr
@@ -468,13 +468,7 @@ end
 class JSONSomething
   include JSON::Serializable

-  property value : JSONAttrValue(Set(JSONSomethingElse)?)?
-end
-
-class JSONSomethingElse
-  include JSON::Serializable
-
-  property value : JSONAttrValue(Set(JSONSomethingElse)?)?
+  property value : JSONSomething?
 end

 describe "JSON mapping" do
@@ -1122,6 +1116,6 @@ describe "JSON mapping" do

   it "fixes #13337" do
     JSONSomething.from_json(%({"value":{}})).value.should_not be_nil
-    JSONSomethingElse.from_json(%({"value":{}})).value.should_not be_nil
+    JSONAttrValue(JSONSomething).from_json(%({"value":{}})).value.should_not be_nil
   end
 end

[HertzDevil]

IIRC this wouldn't reproduce the bug without this patch

[straight-shoota]

No I checked that it actually does reproduce.

[straight-shoota]

I'm still going to merge this as it to move along with 1.8.1.
We can refactor the specs afterwards.

[straight-shoota]

#13358

[kostya]

btw, I think condition :nilable here can be deleted, and only remains :has_default, not checked.