block nav_bar overriding other navbar blocks

[rsa80610]

When extending from _main_header.html, I noticed that {% block nav_bar%} will override other block such as nav_bar_center and nav_custom_menu's content.
After checking the source code, i found that nav_bar's {% endblock %} was placed in the end of the main_header code(in front the tag), causing the overriding problem.

Although it can be fixed by moving {% endblock %} to the end of navbar-nav part, i think that problem should be mentioned.

The whole project does very significant improvements for developing adminlte-template web to me, thanks for the all hard work!