[Haml] Add an error message to discourage people from using haml_tag with =.
[Haml] Add an error message to discourage people from using haml_tag with =.

askl56/sass,saper/sass,Ahnita/sass,briangonzalez/sass,chriseppstein/haml,hcatlin/sass,dg1dd1ngs/haml,Industrys/sass,Andrey-Pavlov/sass,Nzaga/sass,keithpitty/haml,rammy7219/sass,srawlins/sass,DanteKaris/sass,Ahnita/sass,JuanitoFatas/sass,jaroot32/sass,sideci-sample/sideci-sample-haml,zaru/sass,StephanieMak/sass,cmpereirasi/sass,askl56/sass,baiyanghese/sass,modulexcite/sass,xzyfer/sass,srinivashappy/sass,oss-practice/sass,nazar-pc/haml,danielfein/sass,amitsuroliya/sass,dylannnn/sass,Galactix/sass,Industrys/sass,concord-consortium/haml,tangposmarvin/sass,DanteKaris/sass,Andrey-Pavlov/sass,haml/haml,lmetla/sass,Galactix/sass,Iamronan/sass,rammy7219/sass,stewx/sass,chriseppstein/sass,PMArtz92/sass,amatsuda/haml,dylannnn/sass,tenderlove/sass,antonifs/sass,haml/haml,amitsuroliya/sass,danielfein/sass,modulexcite/sass,schneems/sass,StephanieMak/sass,nickg33/sass,stanhu/haml,srawlins/sass,haml/haml,chriseppstein/haml,macressler/haml,Nzaga/sass,Iamronan/sass,MetaAshley/sass,tangposmarvin/haml,nazar-pc/haml,zaru/sass,antonifs/sass,tangposmarvin/sass,Anshdesire/sass,PMArtz92/sass,Anshdesire/sass,cmpereirasi/sass,stanhu/haml,xzyfer/sass,purcell/haml,ridixcr/sass,HugoRLopes/sass,tangposmarvin/haml,OrenBochman/sass,keithpitty/haml,jaroot32/sass,nicklima/sass,saper/sass,OrenBochman/sass,liquidmetal/sass,uniba/hamldoc_ja,nickg33/sass,MetaAshley/sass,hcatlin/sass,nicklima/sass,schneems/sass,14113/sass,ridixcr/sass,liquidmetal/sass,lmetla/sass,stefanpenner/haml,srinivashappy/sass,dg1dd1ngs/haml,JuanitoFatas/sass,stewx/sass,HugoRLopes/sass,macressler/haml,haml/haml