[Bug]: Beam tag is not working properly

### What happened?

Beam Go tag `beam:"name"` is not working with automatic coder.

Example:

```go
type PropertyUpdate struct {
PropertyName string `beam:"property_name"`
Operation operation `beam:"operation"`
Operand *structpb.Value `beam:"operand"
UserID string `beam:"user_id"`
}
```

Failing with:

```
decoding a *struct { Property_name string "beam:\"property_name\""; Operation int64 "beam:\"operation\""; Operand *structpb.Value "beam:\"operand\""; User_id string "beam:\"user_id\"" }
```

The SDK is expecting a field named `Property_name` in the struct while it should expect `PropertyName`.

### Issue Priority

Priority: 3 (minor)

### Issue Components

- [ ] Component: Python SDK
- [ ] Component: Java SDK
- [X] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [ ] Component: Website
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Samza Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner