Case sensitivity

This used to work (1.0.5):

`{"t": 10.0}`

```
type struct F {
  T float64 `json:"T"`
}
```

With 1.1.3, I have to do this:

```
type struct F {
  T float64 `json:"t"`
}
```

With 1.1.3, `{"T":10.0}` works with lowercase t. But the reverse does not work.
