[scala] add [date-time] field to codegen unit test

Created by: sullis

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

modules/openapi-generator/src/test/resources/3_0/scala_reserved_words.yaml

@@ -75,6 +75,9 @@ components:
           type: string
         foobar:
           type: boolean
+        created_at:
+          type: string
+          format: date-time
       required:
         - id
         - try
@@ -82,3 +85,4 @@ components:
         - finally
         - lazy
         - foobar
+        - created_at

modules/openapi-generator/src/test/resources/codegen/scala/SomeObj.scala.txt

@@ -36,7 +36,8 @@ case class SomeObj (
   `lazy`: String,
   `private`: Option[String] = None,
   `type`: Option[String] = None,
-  foobar: Boolean
+  foobar: Boolean,
+  createdAt: DateTime
 ) extends ApiModel
 
 object SomeObjEnums {