Retain line break characters in property values.

When loading a property file like this into the environment, the backslashes are recognised properly and the value is correctly loaded, but the newline characters are missing.
So this property file

    CHANGELOG=* Fixed this. \
     * Fixed that. \
     * Fixed something else.

causes this to be added to the Jenkins environment:

    CHANGELOG=* Fixed this. * Fixed that. * Fixed something else.

This doesn't seem like the correct behaviour, and Jenkins seems to work fine with multi-line environment variables set elsewhere, so this pull request adds the newlines to the value where appropriate.

I added a couple of test cases for this which pass, along with the existing test suite.

We're using this in a couple of jobs, using the newline-containing value with various plugins and it works fine.