Fixed inline renderer destroying newlines

When using inline renderer to diff strings with `split_characters` option turned on, sometimes a multi-line `Horde_Text_Diff_Op_Change` appears. Now when that happens, these lines get joined by `\n` (newline) symbol to inline-diff the actual change. The resulting line gets splitted by characters, meaning a single `\n` symbol is now a line in the view of the inline-diff. This line then gets eliminated ([here](https://github.com/horde/horde/blob/4f619704fce7beeae196caf2d15044ef09685800/framework/Text_Diff/lib/Horde/Text/Diff.php#L198) and [here](https://github.com/horde/horde/blob/4f619704fce7beeae196caf2d15044ef09685800/framework/Text_Diff/lib/Horde/Text/Diff/Engine/Native.php#L33-34)), malforming the result.

For example, when diffing 
```
One
Two
```

and

```
Ones
Twos
```

The result is `OnesTwos` - linebreak missing.