WebCmdlets: WebResponseObject.CoreClr.cs every get on Headers creates a new Dictionary

The Headers property creates a new Dictionary which then gets populated.  Users typically do something like

```powershell
if (obj.Headers.ContainsKey("Something"))
{
    obj.Headers["Something"] ...
}
```

which now instantiates a new Dictionary twice, we should optimize this code.  consider using a `ReadOnlyDictionary`