Breaks if the map key contains "\x" string

Hi,

Given the following snippet:

``` go
package main

import (
    "expvar"
    "net/http"
)

func main() {
    m := expvar.NewMap("test")
    m.Add("\x00", 1)
    http.ListenAndServe(":1234", nil)
}
```

Expvarmon stops showing results all together. You can add a sleep time before the add and observe it only happens when the string is added.
