Elixir - generated code has errors and warnings on latest (1.7) elixir (pr incoming)

Description

Hi @niku - thanks for your library - it's been a great help to me.

When testing it against teamwork.com swagger definition (my fork with fixes to make it compatible with swagger-codegen (changed nil to 0)) the generated code had a couple of (thousand) quoted atom warnings and some api binding errors, with the changes in the incomming PR those errors and warnings will be fixed.

I have also cherry picked my as unmerged PR commit https://github.com/swagger-api/swagger-codegen/pull/12464 as without it, the mvn package task fails to package modules/swagger-codegen-cli/target/swagger-codegen-cli.jar - lets see which gets merged first.

Swagger-codegen version

master (61cfeac3b9d855b4eb8bffa0d118bece117bcb7d)

Swagger declaration file content or url

https://gist.github.com/fcbry/465e767f059504c05717fc715345bf22

Command line used for generation

mvn package -DskipTests  -D"maven.test.skip=true" && java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate \
    -i "C:\Users\BryanHunt\Downloads\teamwork.com-api-object-reference.oas.yml" \
    -l elixir -D"packageName=fc_teamwork" -D"invokerPackage=FC.Teamwork" \
    -o C:\code\fcbry_teamwork_poc\apps\fc_teamwork_client

Steps to reproduce

As above

Related issues/PRs

No

Suggest a fix/enhancement

I'm providing a PR to fix this

Start by running the reported Maven package and Java CLI generation command with the linked Swagger declaration, then inspect the generated Elixir output for the quoted-atom warnings and API binding errors. Compare the results with the incoming fix described in the issue; done means the generated code no longer produces those errors and warnings on Elixir 1.7.