[Golang][client] fix CacheExpires ineffassign, Fix #464

Created by: grokify

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@antihax @bvwells

Description of the PR

fix https://github.com/OpenAPITools/openapi-generator/issues/464

client.go has an ineffectual if statement in the CacheExpires function. This results in a lower score on Go Report Card for the generated code described here: https://github.com/OpenAPITools/openapi-generator/issues/464 . Go Report Card is a commonly used app with it's shield on many GitHub repo READMEs.

Here is an example:

https://goreportcard.com/report/github.com/grokify/go-stackexchange

Two options to resolve this include:

Option 1

lifetime, err := time.ParseDuration(maxAge + "s") 
if err != nil {
    expires = now
} else {
    expires = now.Add(lifetime)
}

Option 2

This option works because an error will return a 0s duration, but it may be better to not rely on this behavior.

lifetime, _ := time.ParseDuration(maxAge + "s")
expires = now.Add(lifetime)

Either approach will resolve the issue on Go Report Card for the generated code.

This PR uses option 1 since it is easier to understand, explicitly handles the error and is similar to other error handling in the function.