Allow `data-background` images to load even if there is whitespace at the end.
Allow `data-background` images to load even if there is whitespace at the end.

Fixes #2032.

gapple/myplanet-revealjs,rbtcollins/reveal.js,denehyg/reveal.js,lecovi/reveal.js,lvh/reveal.js,snowyu/reveal.js,10xEngineer/reveal.js,gapple/myplanet-revealjs,uggla/reveal.js,siddhartha-gadgil/presentations,10xEngineer/reveal.js,stefan-langenmaier/reveal.js,rbtcollins/reveal.js,siddhartha-gadgil/presentations,dbs/reveal.js,Gottox/presentations,maximesinclair/presentations,stevedesmond-ca/presentations,Jean85/reveal.js,jlevallois/Kreveal.js,kenkunz/js-crash-course,fbecart/Presentations,fbecart/Presentations,seefood/DIYHolidays,lvh/reveal.js,rbtcollins/reveal.js,lecovi/reveal.js,rbtcollins/reveal.js,jayramj/agile-retro,marc0der/sdkman-talk,uggla/reveal.js,rajgoel/reveal.js,DiUS/reveal.js,rbtcollins/reveal.js,VisualComputing/Transformations,siddhartha-gadgil/presentations,tehfedaykin/10CommandmentsofCommunityOrganizing,lecovi/reveal.js,rajgoel/reveal.js,Jean85/reveal.js,denehyg/reveal.js,burnpanck/reveal.js,agross/git-reveal,seefood/DIYHolidays,christofmarti/reveal.js,soulcharmer/pubquiz_template,jlevallois/Kreveal.js,stevedesmond-ca/presentations,hakimel/reveal.js,tehfedaykin/10CommandmentsofCommunityOrganizing,christofmarti/reveal.js,VisualComputing/Transformations,soulcharmer/pubquiz_template,kenkunz/js-crash-course,jayramj/agile-retro,marcusdb/funwithfunctional,hakimel/reveal.js,stefan-langenmaier/reveal.js,marc0der/sdkman-talk,burnpanck/reveal.js,maximesinclair/presentations,dbs/reveal.js,agross/git-reveal,rbtcollins/reveal.js,marcusdb/funwithfunctional,snowyu/reveal.js,DiUS/reveal.js