fix bug when path contains directory with space

[aleksandergrzyb]

What?

Uses fileURLWithPath instead of URLWithString.

Why?

For file system paths we should use fileURLWithPath:

For example URLWithString fails if path contains Application Support directory.

[codecov]

Codecov Report

Merging #123 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #123   +/-   ##
=======================================
  Coverage   98.43%   98.43%           
=======================================
  Files          14       14           
  Lines         960      960           
=======================================
  Hits          945      945           
  Misses         15       15           

Impacted Files	Coverage Δ
Sources/SPTPersistentCache.m	98.26% <100.00%> (ø)
Sources/SPTPersistentCacheFileManager.m	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4f4baa7...234c614. Read the comment docs.