Update dropdown.js

[culm]

点击dropdown的下落列表，没法知道点击的是哪个dom,在hide.bs.dropdown的回调中，e.clickTarget代表点击的dom，可以更新dropdown-toggle显示的html

[twbs-savage]

Tests failed. Automated cross-browser testing via Sauce Labs and Travis CI shows that the JavaScript changes in this pull request are: BUSTED

Commit: 2a5ba79
Build details: https://travis-ci.org/twbs-savage/bootstrap/builds/68473340

(Please note that this is a fully automated comment.)

[twbs-savage]

Tests failed. Automated cross-browser testing via Sauce Labs and Travis CI shows that the JavaScript changes in this pull request are: BUSTED

Commit: 1509bed
Build details: https://travis-ci.org/twbs-savage/bootstrap/builds/68473890

(Please note that this is a fully automated comment.)

[cvrebert]

• We aren't accepting new features into Bootstrap v3 at this point, in order to focus on the upcoming Bootstrap v4.
• Your particular use-case should be addressed in v4 by the change mentioned in Have Bootstrap plugin events forward the underlying originalEvent that caused them #15393.
• Corresponding unit tests are required when making changes to Bootstrap's JavaScript; see https://github.com/twbs/bootstrap/tree/master/js/tests for how-to info.
• The Bootstrap Core Team is unable to converse in Chinese at this time. English communications are strongly preferred, although IIRC we are able to accommodate German, Greek, and possibly French, if English isn't feasible.