New cacheDir causes cache to be ignored(?)

Using a cache across client seems broken when using a custom cache directory.

Repro:

In Python:
!mkdir ~/asdfasdfasdf
syn.cache.cache_root_dir='~/asdfasdfasdf'
f = syn.get('syn4994640')

Then in R:
synapseCacheDir('~/asdfasdfasdf')
ent <- synGet('syn4994640')

Or switch the order between R and Python. Either way the file gets downloaded the second time from R/Python

See SYNR-1054