Fix issue #654: Set discriminator for oneOf mapping correctly.

With the changes of PR #620, the generation of the Type-Info-Annotation for the POJO model classes is broken in case there is a oneOf discriminator mapping in the OpenAPI model. This commit re-introduces the correct generation for those annotation.

(cherry picked from commit 3149424c1ea1fbe7447266aeb9b91a1306380900)

Author: Timo Rohrberg

src/main/java/io/swagger/codegen/v3/generators/DefaultCodegenConfig.java

@@ -1376,6 +1376,17 @@ else if (schema instanceof ComposedSchema) {
                     }
                 }
             }
+
+            final List<Schema> oneOf = composed.getOneOf();
+            if (oneOf != null && !oneOf.isEmpty()) {
+                if (schema.getDiscriminator() != null) {
+                    codegenModel.discriminator = schema.getDiscriminator();
+                    if (codegenModel.discriminator != null && codegenModel.discriminator.getPropertyName() != null) {
+                        codegenModel.discriminator.setPropertyName(toVarName(codegenModel.discriminator.getPropertyName()));
+                    }
+                }
+            }
+
             if (parent != null) {
                 codegenModel.parentSchema = parentName;
                 codegenModel.parent = toModelName(parentName);