Use the correct method for #alter_database_privilege test

Whoops.

```  
3) InfluxDB::Client#alter_database_privilege should POST to alter privileges for a user on a database
     Failure/Error: stub_request(:post, "http://influxdb.test:9999/db/foo/users/useruser").write(
     NoMethodError:
       undefined method `write' for #<WebMock::RequestStub:0x7f83f108b340>
```