[BUG] [KOTLIN] Invalid quotation for String default values

[chemist777]

Version: 4.3.0 and master branch.

AbstractKotlinCodegen.java uses invalid quotation for default values of Strings.

It uses single quotes instead of double quotes. Kotlin only supports string literals surrounded by double quotes as plain Java codegen.
It prevents the use of default values in code generation.

Line: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractKotlinCodegen.java#L943

This is a fragment of toDefaultValue method:

...
        } else if (ModelUtils.isStringSchema(p)) {
            if (p.getDefault() != null) {
                return "'" + p.getDefault() + "'";
            }
        }

It should be replaced with:

...
        } else if (ModelUtils.isStringSchema(p)) {
            if (p.getDefault() != null) {
                return "\"" + p.getDefault() + "\"";
            }
        }