Delay timeUnit is defined as a Java's TimeUnit (an enum), but API accepts any string

[JoaoGFarias]

Describe the issue
The Delay timeUnit must the value valid for the Java's standard TimeUnit (code), but the API accepts any string.

Causing MockServer to create a delay without a TimeUnit.

MockServer version
5.15.0

Expected behaviour
MockServer should reject the request without a valid TimeUnit