[REST] /stats/proj/gcc2/iter/4.9.0/doc/{docId} failed to handle the file with directory

Description of problem:
Zanata convert forward slash '/' into comma ','
Converted URL, however, did not work with REST API
/stats/proj/gcc2/iter/4.9.0/doc/{docId}

which is for getting document statistics

and returned 404

Version-Release number of selected component (if applicable):
Zanata 3.5.1

How reproducible:
Always

Steps to Reproduce:
1. Run

curl -k -v -X GET -H "X-Auth-User:${ZANATA_USERNAME}" -H "X-Auth-Token:${ZANATA_KEY}" -H "Content-Type:application/json" "https://fedora.zanata.org/rest/stats/proj/freeipa/iter/master/doc/install%2Fpo%2Fipa

Actual results:
404 Not Found

Expected results:
200 Ok

Additional info: