[Golang][client] fix go import issue

[anandshah05]

PR checklist

Adding optional import for non primitive fields

Description of the PR

#1082

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[wing328]

cc @antihax (2017/11) @bvwells (2017/12) @grokify (2018/07)

[anandshah05]

Please merge

[wing328]

@anandshah05 I did some tests locally and it's adding the following for the test spec you provided:

@@ -16,6 +16,7 @@ import (
        "net/url"
        "strings"
        "fmt"
+       "github.com/antihax/optional"
 )

I'm ok with the change.

[anandshah05]

Thank you!

[wing328]

@anandshah05 thanks again for the PR, which is included in the v3.3.0 minor release: https://twitter.com/oas_generator/status/1046941449609068544