Redirect URL not working for GitHub provider

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Hi there,
I'm trying to get GitHub authentication working at the moment, but I'm running into an issue where the provider is seemingly not setting the `redirect_uri` query parameter when redirecting to GitHub's `authorize` endpoint. I think I am configuring it correctly and I double-checked that my redirect URL is set to localhost, but I still get redirected to the prod domain.

```go
var (
ghClientId = os.Getenv("OAUTH_GITHUB_CLIENT_ID")
ghClientSecret = os.Getenv("OAUTH_GITHUB_CLIENT_SECRET")
ghRedirectUrl = os.Getenv("OAUTH_GITHUB_REDIRECT_URL")
)

func main() {
// ...

goth.UseProviders(
github.New(ghClientId, ghClientSecret, ghRedirectUrl, "email"),
)

// ...
}
```

When I check the redirects in my browser's developer tools it's visible that the `redirect_uri` parameter is missing entirely.

Start at the GitHub provider configured through github.New and trace how its authorization URL is built. Reproduce the redirect with browser developer tools, then verify that the configured localhost URL is included as the redirect_uri parameter in the GitHub authorize request.

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