chore(doc-gen): fix blank line trimming for Jade docs

Closes angular#4217

Author: petebacondarwin

docs/angular.io-package/rendering/trimBlankLines.js

@@ -1,9 +1,9 @@
-module.exports = function(encodeCodeBlock) {
+module.exports = function() {
   return {
     name: 'trimBlankLines',
     process: function(str) {
       var lines = str.split(/\r?\n/);
-      while(lines[0] === '') {
+      while(lines.length && (lines[0].trim() === '')) {
         lines.shift();
       }
       return lines.join('\n');

docs/angular.io-package/rendering/trimBlankLines.spec.js

@@ -0,0 +1,18 @@
+var factory = require('./trimBlankLines');
+
+describe('trimBlankLines filter', function() {
+  var filter;
+
+  beforeEach(function() {
+    filter = factory();
+  });
+
+  it('should be called "trimBlankLines"', function() {
+    expect(filter.name).toEqual('trimBlankLines');
+  });
+
+  it('should remove all empty lines from the start of the string', function() {
+    expect(filter.process('\n\n\nsome text\n\nmore text\n\n'))
+        .toEqual('some text\n\nmore text\n\n');
+  });
+});