Passing SafeString as content

Doesn't work:

```
> pithy.button('.button', new pithy.SafeString('string'))
g.SafeString {_value: "", toString: function, valueOf: function}
```

Works:

```
> pithy.button('.button', [new pithy.SafeString('string')])
g.SafeString {_value: "string", toString: function, valueOf: function}
```

Reproduce the direct and array-wrapped SafeString examples from the issue, then inspect the pithy.button entry point and its SafeString content handling. The fix is complete when passing a SafeString directly produces the same button content as passing it in an array, with regression coverage for both forms.

Written by the indexing model from the issue text.