[all] Fix NullPointerException with list of simple types

Created by: jmini

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 and ./bin/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, 4.0.x. Default: master.
• Copied the technical committee: @OpenAPITools/generator-core-team (change to DefaultCodegen)

Description of the PR

Fixes #677 (closed)

There is a null pointer for when request body is an array of items that are simple-types:

      requestBody:
        content:
          application/json:
            schema:
              type: array
              items:
                type: string

This PR set the name to request_body for those cases (as it is the case for map). A Unit Test is added.

---

Stacktrace before the fix:

  Exception: null
	at org.openapitools.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:942)
	at org.openapitools.codegen.DefaultGenerator.processPaths(DefaultGenerator.java:836)
	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:463)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:794)
	at org.openapitools.codegen.cmd.Generate.run(Generate.java:315)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:59)
Caused by: java.lang.NullPointerException
	at org.openapitools.codegen.DefaultCodegen.underscore(DefaultCodegen.java:3129)
	at org.openapitools.codegen.languages.PerlClientCodegen.toVarName(PerlClientCodegen.java:279)
	at org.openapitools.codegen.languages.PerlClientCodegen.toParamName(PerlClientCodegen.java:292)
	at org.openapitools.codegen.DefaultCodegen.toArrayModelParamName(DefaultCodegen.java:795)
	at org.openapitools.codegen.DefaultCodegen.fromRequestBody(DefaultCodegen.java:4395)
	at org.openapitools.codegen.DefaultCodegen.fromOperation(DefaultCodegen.java:2329)
	at org.openapitools.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:910)
... 5 more