Use url-safe base64 encoding for download-shared-object api

[cesnietor]

fixes: #3304

Uses URLEncoding instead of StdEncoding to ensure url-safe strings.
Includes test.

Test steps:

With an existing bucket, folder and objects.

• With different filenames e,.g. with .jpg, .svg, .png, spaces, etc
• Click share file
• Copy and paste the new url in the browser
• The url should contain same url as Object browser
• Object should be downloaded
• Test injecting wrong urls (base64encoded)
• Should return error or forbidden errors if url doesn't point to internal MinIO server

[bayasdev]

Perfect

[allanrogerr]

LGTM - tested