StringValueImpl and RawStringValueImpl does not satisfy equals/hashCode laws

version 0.7.0-p2

``` scala
val str = "a"
val a1 = ValueFactory.newRawString(str.getBytes("UTF-8"))
val a2 = ValueFactory.newString(str)
println(a1 == a2) // true
println(a1.hashCode) // 128
println(a2.hashCode) // 97
```

`a1.hashCode` should be same as `a2.hashCode` if `a1.equals(a2)`
