Fixed malformed Data URI

Removed a space from the form-select mixin that was causing Ruby's
URI.parse method to throw an error. Can't 100% guarantee that spaces in
data URIs are actually against spec, but my reading of RFC 2397 suggests
they aren't, and it'll decode the same without the space anyway.

Ran into the issue when using a gem (sinatra-assetpack) that attempts to
parse URLs in CSS to see if they can be "cache busted".