Add metadata to existing coupon

Hello, I try with the following and I get no errors
```ruby
@wc_api_client.put("coupons/42", { meta_data: [{ key: "email", value: "email@example.com" }] })
```
but then nothing changes on the Coupon
```ruby
@wc_api_client.get("coupons/42")['meta_data']
=> nil
```
What am I doing wrong?