BUG: JSON decodes array of structs wrong

The JSON parser of the hcl package decodes two elements for the inner array instead of one:

```json
{"x":[{"a":1, "b":2}]}
```

Here is the Go test to verify this:

```go
// === RUN TestArrayWithStruct
// === RUN TestArrayWithStruct/encoding/json
// === RUN TestArrayWithStruct/hashicorp/hcl
// --- FAIL: TestArrayWithStruct (0.00s)
// --- PASS: TestArrayWithStruct/encoding/json (0.00s)
// --- FAIL: TestArrayWithStruct/hashicorp/hcl (0.00s)
// main_test.go:38: got main.Y{X:[]main.X{main.X{A:1, B:0}, main.X{A:0, B:2}}} want main.Y{X:[]main.X{main.X{A:1, B:2}}}
func TestArrayWithStruct(t *testing.T) {
type X struct {
A, B int
}
type Y struct {
X []X
}

j := `{"x":[{"a":1,"b":2}]}`
y := Y{X: []X{{A: 1, B: 2}}}
t.Run("encoding/json", func(t *testing.T) {
var v Y
if err := json.Unmarshal([]byte(j), &v); err != nil {
t.Fatalf("got error %v want nil", err)
}
if got, want := v, y; !reflect.DeepEqual(got, want) {
t.Fatalf("got %#v want %#v", got, want)
}
})
t.Run("hashicorp/hcl", func(t *testing.T) {
var v Y
if err := hcl.Decode(&v, j); err != nil {
t.Fatalf("got error %v want nil", err)
}
if got, want := v, y; !reflect.DeepEqual(got, want) {
t.Fatalf("got %#v want %#v", got, want)
}
})
}
```

Start by running the provided Go TestArrayWithStruct reproduction against hcl.Decode and inspect the HCL JSON decoding path it exercises. The fix is done when the array contains one struct with both A and B populated and the regression test passes.

Written by the indexing model from the issue text.