fix(module:page-header): auto title doesn't change when route changed.

[taogeT]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ng-alain/delon/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

When parameter: autoTitle is true, the title text doesn't change if router changed.

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[cipchk]

抱歉，我完全不理解这个修改的出发点是什么？是否有一个明确的重现示例，以及配对的测试用例呢？

[taogeT]

已经改进了测试用例，测试自动渲染的breadcrumb & title在路由变更时也会及时发生变化。

其实这次和上次的pull request类似，都是解决无参数<page-header></page-header>情况下，当路由变化时自动渲染的元素也要对应路由改变。上次只修正了breadcrumb不变的情况，这次是解决title不变的问题。

添加的property: _titleText，是用来保留[title]的赋值。原来的写法 _title 的值无法区分是由[title] string 得到的还是 [autoTitle] = true 自动赋值的。

[cipchk]

非常感谢，但是你的逻辑有点让我看得晕晕的，我已经在 #296 一并修复这一现象。