Error occurs while build generated typescript

Description

On use the header menu item: Generate Client/typescript-angular all the definitions... All the definitions types with type:"string" and format: "date-time" returns in the generated models (into the models folder) a model with type "Date" and it should returns "string".

Swagger-codegen version

Version 4.0.1

Swagger declaration file content or url

birthdate:
 properties:
  type: string
  format: date```

Generates a model .ts: 
```ts
birthDate: string; // Well done!

birthdate:
 properties:
  type: string
  format: date-time

Generates a model .ts:

birthDate: Date; // Wrong!

The format property isn't a Json primitive.

Steps to reproduce

1. Declare a property with type: string and format: date
2. Declare another property with type: string and format date-time
3. Go to Menu /Generate Client / typescript-angular
4. Open zip file and find into the /models folder the generated type is (string for date // Well and Date for date-time // Wrong)

Start by reproducing the issue with the two YAML property definitions and inspect the typescript-angular generator path that produces files in the models folder. Compare the generated types for format: date and format: date-time, then verify the corrected output against both cases.