Spacing issue in content:encoded field of rss2

I have a post content written in markdown like this
```
This is a first line of text
then I start a new line of text.
```

When rss2 file is created the `content:encoded` contains:

```
This is a first line of textthen I start a new line of text.
```

Note missing space between the first and the second line.

In my case I fixed it by using a custom template that instead of
```
post.content | noControlChars | safe
```

renders
```
post.content | replace("\n", " ") | noControlChars | safe
```

What do you think about making it a default behavior of `noControlChars` filter though?

Start by locating the rss2 template and the noControlChars filter used by post.content. Reproduce the issue with the two-line Markdown example, then verify that generated content:encoded preserves a separator between lines and add or update the relevant feed-generation test if one exists.

Written by the indexing model from the issue text.