handle '/' prefixed object name properly

[balamurugana]

When below is tried, minio-go reties 5 times unnecessarily.

$ mc cp README.md local/rawphoto//1503389301.18356.dng
README.md:                      32.31 KB / 6.46 KB ┃▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┃ 500.00% 3.14 KB/s -8s
mc: <ERROR> Failed to copy `README.md`. Object name contains unsupported characters.
mc: <ERROR> Session safely terminated. To resume session `mc session resume UjRdjJzO`

At server side, below is the log

Drive Capacity: 77 GiB Free, 234 GiB Total
ERRO[1450] Unable to create an object. /rawphoto//1503389301.18356.dng  cause=Object name invalid: rawphoto#/1503389301.18356.dng source=[object-handlers.go:566:objectAPIHandlers.PutObjectHandler()] stack=object-api-input-checks.go:146:checkPutObjectArgs fs-v1.go:533:fsObjects.PutObject <autogenerated>:421:(*fsObjects).PutObject object-handlers.go:544:objectAPIHandlers.PutObjectHandler api-router.go:60:(objectAPIHandlers).PutObjectHandler-fm
ERRO[1451] Unable to create an object. /rawphoto//1503389301.18356.dng  cause=Object name invalid: rawphoto#/1503389301.18356.dng source=[object-handlers.go:566:objectAPIHandlers.PutObjectHandler()] stack=object-api-input-checks.go:146:checkPutObjectArgs fs-v1.go:533:fsObjects.PutObject <autogenerated>:421:(*fsObjects).PutObject object-handlers.go:544:objectAPIHandlers.PutObjectHandler api-router.go:60:(objectAPIHandlers).PutObjectHandler-fm
ERRO[1453] Unable to create an object. /rawphoto//1503389301.18356.dng  cause=Object name invalid: rawphoto#/1503389301.18356.dng source=[object-handlers.go:566:objectAPIHandlers.PutObjectHandler()] stack=object-api-input-checks.go:146:checkPutObjectArgs fs-v1.go:533:fsObjects.PutObject <autogenerated>:421:(*fsObjects).PutObject object-handlers.go:544:objectAPIHandlers.PutObjectHandler api-router.go:60:(objectAPIHandlers).PutObjectHandler-fm
ERRO[1456] Unable to create an object. /rawphoto//1503389301.18356.dng  cause=Object name invalid: rawphoto#/1503389301.18356.dng source=[object-handlers.go:566:objectAPIHandlers.PutObjectHandler()] stack=object-api-input-checks.go:146:checkPutObjectArgs fs-v1.go:533:fsObjects.PutObject <autogenerated>:421:(*fsObjects).PutObject object-handlers.go:544:objectAPIHandlers.PutObjectHandler api-router.go:60:(objectAPIHandlers).PutObjectHandler-fm
ERRO[1460] Unable to create an object. /rawphoto//1503389301.18356.dng  cause=Object name invalid: rawphoto#/1503389301.18356.dng source=[object-handlers.go:566:objectAPIHandlers.PutObjectHandler()] stack=object-api-input-checks.go:146:checkPutObjectArgs fs-v1.go:533:fsObjects.PutObject <autogenerated>:421:(*fsObjects).PutObject object-handlers.go:544:objectAPIHandlers.PutObjectHandler api-router.go:60:(objectAPIHandlers).PutObjectHandler-fm

Refer #764

[balamurugana]

Refer minio/minio#4840