can't specify multiple schemes for openapiv2_swagger

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**protobuff like this**
```
option (grpc.gateway.protoc_gen_swagger.options.openapiv2_swagger) = {
info: {
title: "Example code"
version: "1.0"
contact: {
name: "project"
url: "https://github.com/codesprut/project"
email: "email@gmail.com"
}
}
schemes: HTTP
schemes: HTTPS
consumes: "application/json"
produces: "application/json"
}
```

**Produce**:
```
Error: Duplicate option map key schemes
at onoptionMap (\node_modules\protocol-buffers-schema\parse.js:459:43)
at onoption (\node_modules\protocol-buffers-schema\parse.js:421:19)
at Function.parse (\node_modules\protocol-buffers-schema\parse.js:664:19)
```

Start in parse.js at the onoptionMap and onoption entry points named by the error, using the provided openapiv2_swagger schema as the reproduction. Confirm the parser's expected handling of repeated schemes and verify that both HTTP and HTTPS can be specified without the duplicate-key error.

Written by the indexing model from the issue text.