meta utf8 in html is bad

In the generated documents the header contains:

```
<meta http-equiv='content-type' value='text/html;charset=utf8'>
```

it should be:

```
<meta http-equiv='content-type' content='text/html;charset=utf8'>
# content vs value
```

This makes browser generate bad characters for the extended characters like "é è à ..."

This bug is not present when ronn is ran in server mode "-S" option
