Fix the cache lookup in _info

[YeahItsMeAgain]

The info function retrieved the fullpath from the cache in its initial lookup but used the given path (which is a user given variable) on the second instance.

Fixed it to use fullpath both times.

You can reproduce the bug with the following code:

# Filling the cache.
fs.isdir("/bucket")

# The cache seems ok.
print(fs.dircache._cache)

# The cache "miss" happens now because the key doesn't start with "/".
print(fs.info("/bucket/file"))

[martindurant]

Since you already have a test in the PR description, can you please add this to the code to make sure we don't accidentally undo this sometime?

[YeahItsMeAgain]

Sure, I changed the existing test :)