Fix prescan bug: Second and invalid http-equiv

```
#data
<meta http-equiv="refresh" http-equiv="Content-Type" content="text/html; charset=iso8859-2">
#encoding
None
```

The spec uses `attribute list`, to keep track of parsed attribute names,
discarding duplicates.

While `html5lib._inputstream.EncodingParser` returns: 'iso-8859-2'.