In some cases, images in the media library may contain spaces in
their filenames. This results in an invalid `srcset` attribute,
causing broken images on the front end. This change fixes the issue
by replacing spaces in URLs with URL encoded '%20' characters before
returning the `srcset` string.
Props underdude, joemcgill.
Fixes#36549.
git-svn-id: https://develop.svn.wordpress.org/trunk@38052 602fd350-edb4-49c9-b593-d223f7449a82