[Elixir] Allow Elixir deserialization whenever the response is success

[yknx4]

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, ./bin/security/{LANG}-petstore.sh and ./bin/openapi3/security/{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\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.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

• Define macro that return true when response is success (Status Code 2xx)
• Guard functions that are hardcoded to status 200

closes #2334
cc @mrmstn

[wing328]

@yknx4 the CI job failed with the following error messages:

warning: found quoted atom "id" but the quotes are not required. Atoms made exclusively of Unicode letters, numbers, underscore, and @ do not require quotes
  lib/openapi_petstore/model/user.ex:23

warning: found quoted atom "username" but the quotes are not required. Atoms made exclusively of Unicode letters, numbers, underscore, and @ do not require quotes
  lib/openapi_petstore/model/user.ex:24

warning: found quoted atom "firstName" but the quotes are not required. Atoms made exclusively of Unicode letters, numbers, underscore, and @ do not require quotes
  lib/openapi_petstore/model/user.ex:25

warning: found quoted atom "lastName" but the quotes are not required. Atoms made exclusively of Unicode letters, numbers, underscore, and @ do not require quotes
  lib/openapi_petstore/model/user.ex:26

warning: found quoted atom "email" but the quotes are not required. Atoms made exclusively of Unicode letters, numbers, underscore, and @ do not require quotes
  lib/openapi_petstore/model/user.ex:27

warning: found quoted atom "password" but the quotes are not required. Atoms made exclusively of Unicode letters, numbers, underscore, and @ do not require quotes
  lib/openapi_petstore/model/user.ex:28

warning: found quoted atom "phone" but the quotes are not required. Atoms made exclusively of Unicode letters, numbers, underscore, and @ do not require quotes
  lib/openapi_petstore/model/user.ex:29

warning: found quoted atom "userStatus" but the quotes are not required. Atoms made exclusively of Unicode letters, numbers, underscore, and @ do not require quotes
  lib/openapi_petstore/model/user.ex:30


== Compilation error in file lib/openapi_petstore/request_builder.ex ==
** (CompileError) lib/openapi_petstore/request_builder.ex:122: cannot find or invoke local is_success/1 inside guard. Only macros can be invoked in a guard and they must be defined before their invocation. Called as: is_success(status)

[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.2.1:exec (compile) on project ElixirPetstoreClientTests: Command execution failed.: Process exited with an error: 1 (Exit value: 1) -> [Help 1]

Please take a look when you've time.

Please also resolve the merge conflicts. Let us know if you need help on that.

[wing328]

I think this PR is longer needed as #2355 has been merged into master