Revert "fix whitespace encoding in urls"

[w00fz]

Reverts #3719

The code change from #3719 introduces a breaking change (as in, literally breaking) and throws a fatal error.

Culprit code:

$excerpt['element']['attributes']['src'] = str_replace(' ', '%20', $excerpt['element']['attributes']['src']);

This lacks sanity checks around the src presence under attributes, which throws a fatal error Undefined array key "src".

/cc @dirkjf - Please recreate the PR with adequate checks and unit testing so we don't run into this again, thanks!