fix(MdMenu): fix close-on-select props

[VdustR]

check whether clicked target is contained in menu content element within bodyClickObserver

I found that this.$el is always <!-- --> here. Add a statement isMenuContentEl to prevent menu closing from bodyClickObserver.

MdMenu, MdSelect, MdAutocomplete tested.

Note that md-close-on-select only fire close event fired while there are one or more interaction events bound on md-menu-item

fix #1279