Weird behavior when mixing backends

Some examples of the weird behavior when mixing flag and envvar:

❌ bar default overwrite bar value from envvar
```
$ BAR=bar-envvar ./test-confita -foo=foo-flag
2019/09/29 10:24:37 cfg.Foo: 'foo-flag'
2019/09/29 10:24:37 cfg.Bar: 'bar-default'
```

✅ foo value from envvar it's overwritten by flag as expected
```
$ FOO=foo-envvar ./test-confita -foo=foo-flag
2019/09/29 10:29:22 cfg.Foo: 'foo-flag'
2019/09/29 10:29:22 cfg.Bar: 'bar-default'
```

❌ foo keeps the value from envvar ignoring the flag
```
$ BAR=bar-envvar FOO=foo-envvar ./test-confita -foo=foo-flag
2019/09/29 10:25:33 cfg.Foo: 'foo-envvar'
2019/09/29 10:25:33 cfg.Bar: 'bar-envvar'
```

go code:

```go
package main

import (
"context"
"log"

"github.com/heetch/confita"
"github.com/heetch/confita/backend/env"
"github.com/heetch/confita/backend/flags"
)

type Config struct {
Bar string `config:"bar"`
Foo string `config:"foo"`
}

func main() {
cfg := Config{
Bar: "bar-default",
}

loader := confita.NewLoader(
env.NewBackend(),
flags.NewBackend(),
)

err := loader.Load(context.Background(), &cfg)
if err != nil {
panic(err)
}

log.Printf("cfg.Foo: '%s'\n", cfg.Foo)
log.Printf("cfg.Bar: '%s'\n", cfg.Bar)
}
```

Start by running the provided Go reproduction and trace confita.NewLoader with env.NewBackend and flags.NewBackend to determine backend precedence. Done means the mixed BAR/FOO cases produce consistent precedence without regressions; the payload names no repository files or tests, so locate those before editing.

Written by the indexing model from the issue text.