fix(apigateway): deployment fails when Model's contentType is not spe…

…cified (#6199)

CloudFormation and hence the CDK has marked this property as optional,
however, it is not. CloudFormation deployment will fail if this property
is not specified.

Use 'application/json' as the default as this is the most common case
when specifying the content type of the RestApi Model.

fixes #6161

packages/@aws-cdk/aws-apigateway/lib/model.ts

@@ -61,7 +61,7 @@ export interface ModelOptions {
    * The content type for the model. You can also force a
    * content type in the request or response model mapping.
    *
-   * @default -
+   * @default 'application/json'
    */
   readonly contentType?: string;
 
@@ -167,7 +167,7 @@ export class Model extends Resource implements IModel {
     const modelProps: CfnModelProps = {
       name: this.physicalName,
       restApiId: props.restApi.restApiId,
-      contentType: props.contentType,
+      contentType: props.contentType ?? 'application/json',
       description: props.description,
       schema: util.JsonSchemaMapper.toCfnJsonSchema(props.schema)
     };

packages/@aws-cdk/aws-apigateway/test/test.model.ts

@@ -32,7 +32,8 @@ export = {
         title: "test",
         type: "object",
         properties: { message: { type: "string" } }
-      }
+      },
+      ContentType: 'application/json'
     }));
 
     test.done();