path in caching backends should be absolute

[wch]

For example, with cache_filesystem, if path is relative and the working directory changes, then the caching operations will happen in the wrong directory.

[jimhester]

Agreed, I even suggested this in #51 (comment), but apparently never did anything about it :)

[xhdong-umd]

If I'm understanding correctly, https://github.com/xhdong-umd/memoise/commit/b237c97f8f68e2b3aa44351cbaeb8482bf828db1 should work. I run the tests in package and all passed.

?normalizePath mentioned about this, but I'm not sure if this will cause a problem.

The canonical form of paths may not be what you expect. For example, on macOS absolute paths such as ‘/tmp’ and ‘/var’ are symbolic links.

If you think it's OK I can create a pull request.

[jimhester]

Just using normalizePath is ok, thanks for offering to do a pull request!