[BUG] [JAVA]String type parameter in post request body was generated as Java class type

Created by: simonsonzhang

used cli to create a client sdk jar file: java -jar openapi-generator-cli-5.0.0-beta3.jar generate --input-spec synopsys-coverity-coverity-connect-2020.12-resolved.yaml -g java -o connectAPIclient-2020.12 .

in the swagger yaml file, two String parameters (FilterKey and FilterMatchMode )were defined in the post request body. the only difference is FilterMatchMode is a enum string, it only allows two values ("oneOrMoreMatch" or "noneMatch"), but in the auto Generated java code, the FilterKey was generated as a string type, but FilterMatchMode was generated as a FilterMatchMode class type, both should be generated as String type.

Swagger definition: `

FilterKey string

FilterMatchMode string values: oneOrMoreMatch: (Default) At least one of the values must match the item. noneMatch: None of the values may match the item. Enum:[ oneOrMoreMatch, noneMatch ]

Auto generated JAVA code: @javax.annotation.Generated(value = "io.swagger.codegen.v3.generators.java.JavaClientCodegen", date = "2020-12-12T03:14:15.736Z[GMT]") public class FilterElement { @SerializedName("filterKey") private String filterKey = null;

@SerializedName("matchMode") private FilterMatchMode matchMode = null; `