FIX: [constructor] bbcode would cause markdown crash

app/assets/javascripts/pretty-text/engines/discourse-markdown-it.js.es6

@@ -50,7 +50,9 @@ class Ruler {
 
   getRuleForTag(tag) {
     this.ensureCache();
-    return this.cache[tag];
+    if (this.cache.hasOwnProperty(tag)) {
+      return this.cache[tag];
+    }
   }
 
   ensureCache() {

spec/components/pretty_text_spec.rb

@@ -155,6 +155,10 @@ describe PrettyText do
       expect(cooked).to eq(html.strip)
     end
 
+    it "handles bbcode edge cases" do
+      expect(PrettyText.cook "[constructor]\ntest").to eq("<p>[constructor]<br>\ntest</p>")
+    end
+
     it "can handle quote edge cases" do
       expect(PrettyText.cook("[quote]abc\ntest\n[/quote]")).not_to include('aside')
       expect(PrettyText.cook("[quote]  \ntest\n[/quote]  ")).to include('aside')