Fix go multipart formdata without file

[contribu]

PR checklist

• [x] Read the contribution guidelines.
• [x] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [x] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [x] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Changes

• Golang client
  • Add the boundary to the multipart/form-data content-type header anytime.

Problem

In golang client, the boundary was added to the multipart/form-data content-type header only when the form-data contains files. The multipart/form-data content-type header without the boundary is invalid. ( https://stackoverflow.com/questions/43932900/multipart-form-data-without-a-boundary ) Actually some http servers (e.g. Rack) couldn't handle such a header. So, we added the boundary to the multipart/form-data content-type header anytime.

Technical Committee of Go

no one