Fix query string builder when push_v() is called first (#531)

```rust
query.push_v("uploads");
query.push_kv("x-id", "CreateMultipartUpload");
```

should build `?uploads&x-id=CreateMultipartUpload` instead of
`?uploads?x-id=CreateMultipartUpload`.

Co-authored-by: Russell Cohen <rcoh@amazon.com>