Adds default_path, if it's a directory, to LOAD_PATH
Merge pull request #831 from fj/topic/include-default-path-as-load-path

Adds --default-path, if specified, into LOAD_PATH
Add changelog entry for #831.

[ci skip]
Prefer actually testing load path mutation over mocking.

There wasn't anything testing the logic in `add_project_paths`, so
this approach will be a better guard against regressions.