fix whitespace that precedes the escape in a multiline string.

While working on #2167 I noticed that whitespace preceding the escape character would get swallowed. Example:

```
FROM foo
DESCRIPTION "he is a horse of \
course of \
course"
```

The `description` line would say:

```
he is a horse ofcourse ofcourse
```

This patch makes it so the whitespace is normal preceding the escape, but swallowed after (+ newline). Tests included.