Format:date with example renders time in addition to date

Q&A (please complete the following information)

• OS: Windows
• Browser: Chrome
• Version: 72
• Method of installation: visual studio code extension (openapi-designer)
• Swagger-UI version: [e.g. 3.21.0]
• Swagger/OpenAPI version: OpenAPI 3.0

Content & configuration

I use a visual studio extension to render things on the fly (openapi-designer) and have checked that it uses the latest version of swagger-ui

Example Swagger/OpenAPI definition:

startDate:
   type: string
   format: date
   example: 2018-12-25

Describe the bug you're encountering

There was recently a pull request to make automatic example if it is not present. That works fine, but when I have my own example, then the rendered date for some reason includes timestamp as well:

"startDate": "2018-12-25T00:00:00.000Z",

To reproduce...

1. Render YAML with type: string and format: date
2. Open an example value in the browser

Expected behavior

Expected to see only date in example

"startDate": "2018-12-25",

Start by reproducing the issue with the provided OpenAPI YAML in Swagger UI, following the listed browser steps. Trace the example rendering path for a string with format: date and compare the output with the supplied expected JSON; done means the custom example renders as 2018-12-25 without a timestamp.