math/big: ParseFloat returns err == EOF on success

``` go
package main

import (
    "fmt"
    "math/big"
)

func main() {
    _, _, err := big.ParseFloat("3", 10, 350, big.ToNearestEven)
    fmt.Println(err)
}
```

yields `EOF`.

(I know that `big.Float` is not yet complete. Just filing things as I encounter them so they don't get lost.)
