add case for "multi" to golang

[adamgreenhall]

PR checklist

• Read the contribution guidelines.
• 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\.
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Description of the PR

This PR is to add a missing case for multi (array) properties. For example, in this api:

  /api:
    get:
      parameters:
      - name: ids
        in: query
        required: true
        style: form
        explode: true
        schema:
          type: array
          items:
            minimum: 1
            type: integer

When passed multiple integers (eg [1,2,3,4,5]), the current go code sends off the param value 12345. This PR makes the value 1,2,3,4,5.

[adamgreenhall]

cc @antihax