Unless we want multiple Content-Type headers, Set them rather than Add()ing
Unless we want multiple Content-Type headers, Set them rather than Add()ing

(Also restore myteriously missing portion of previous commit)

postfix/boulder,rolandshoemaker/boulder,deserted/boulder,kuba/boulder,patf/boulder,kuba/boulder,rolandshoemaker/boulder,josephyzhou/boulder,ZCloud-Firstserver/boulder,KyleChamberlin/boulder,benileo/boulder,r0ro/boulder,modulexcite/boulder,mommel/boulder,modulexcite/boulder,ibukanov/boulder,ZCloud-Firstserver/boulder,KyleChamberlin/boulder,mommel/boulder,jcjones/boulder,bifurcation/boulder,jcjones/boulder,josephyzhou/boulder,tomclegg/boulder,jgillula/boulder,lmcro/boulder,letsencrypt/boulder,jmhodges/boulder,postfix/boulder,patf/boulder,deserted/boulder,modulexcite/boulder,KyleChamberlin/boulder,jfrazelle/boulder,julienschmidt/boulder,bmw/boulder,kuba/boulder,letsencrypt/boulder,mozmark/boulder,jsha/boulder,rolandshoemaker/boulder,jcjones/boulder,mozmark/boulder,r0ro/boulder,patf/boulder,ZCloud-Firstserver/boulder,ZCloud-Firstserver/boulder,mehmooda/boulder,postfix/boulder,bifurcation/boulder,bmw/boulder,tomclegg/boulder,modulexcite/boulder,josephyzhou/boulder,bifurcation/boulder,kuba/boulder,lmcro/boulder,benileo/boulder,jgillula/boulder,hlandauf/boulder,josephyzhou/boulder,mozmark/boulder,julienschmidt/boulder,ibukanov/boulder,postfix/boulder,mehmooda/boulder,hlandauf/boulder,jfrazelle/boulder,tomclegg/boulder,hlandauf/boulder,mehmooda/boulder,jfrazelle/boulder,devpaul/boulder,jmhodges/boulder,KyleChamberlin/boulder,jgillula/boulder,mommel/boulder,deserted/boulder,devpaul/boulder,sophie-h/boulder,rolandshoemaker/boulder,lmcro/boulder,lucasallan/boulder,bmw/boulder,jcjones/boulder,lucasallan/boulder,sophie-h/boulder,devpaul/boulder,jmhodges/boulder,hlandauf/boulder,ibukanov/boulder,mehmooda/boulder,jfrazelle/boulder,r0ro/boulder,tomclegg/boulder,patf/boulder,r0ro/boulder,julienschmidt/boulder,jmhodges/boulder,sophie-h/boulder,letsencrypt/boulder,jsha/boulder,mommel/boulder,benileo/boulder,benileo/boulder,lucasallan/boulder,devpaul/boulder,lmcro/boulder,jsha/boulder,julienschmidt/boulder,deserted/boulder,jgillula/boulder,mozmark/boulder