Fix for ':' in headers

Author: cadorn

lib/markdown.js

@@ -1030,7 +1030,7 @@ Markdown.dialects.Maruku = Markdown.subclassDialect( Markdown.dialects.Gruber );
 
 Markdown.dialects.Maruku.block.document_meta = function document_meta( block, next ) {
   // we're only interested in the first block
-  if ( block.lineNumber > 1 ) return undefined;
+  if ( typeof block.lineNumber === "undefined" || block.lineNumber > 1 ) return undefined;
 
   // document_meta blocks consist of one or more lines of `Key: Value\n`
   if ( ! block.match( /^(?:\w+:.*\n)*\w+:.*$/ ) ) return undefined;