Ensure that a heading is preceded by a whitespace

Currently `#Foo` gets converted to `<h1>Foo</h1>` which is invalid. Only `# Foo` should be converted to  `<h1>Foo</h1>`

Author: jhollingworth

lib/marked.js

@@ -15,7 +15,7 @@ var block = {
   code: /^( {4}[^\n]+\n*)+/,
   fences: noop,
   hr: /^( *[-*_]){3,} *(?:\n+|$)/,
-  heading: /^ *(#{1,6}) *([^\n]+?) *#* *(?:\n+|$)/,
+  heading: /^ *(#{1,6}) +([^\n]+?) *#* *(?:\n+|$)/,
   nptable: noop,
   lheading: /^([^\n]+)\n *(=|-){2,} *(?:\n+|$)/,
   blockquote: /^( *>[^\n]+(\n(?!def)[^\n]+)*\n*)+/,