merge heading lines

markdown headings must be a single line

IPython/html/static/notebook/js/textcell.js

@@ -515,9 +515,8 b' var IPython = (function (IPython) {'
     HeadingCell.prototype.render = function () {
         if (this.rendered === false) {
             var text = this.get_text();
-            // Markdown headings must be a single line,
-            // truncate invalid input
-            text = text.split('\n')[0];
+            // Markdown headings must be a single line
+            text = text.replace(/\n/g, ' ');
             if (text === "") { text = this.placeholder; }
             text = Array(this.level + 1).join("#") + " " + text;
             text = IPython.mathjaxutils.remove_math(text);