By default, SpringApplication attempts to deduce the application class
by looking for a main method in the stack. This does not work when
the application is launched by a servlet container via
SpringBootServletInitializer as there's either no main method in the
stack, or the main method is that of the servlet container, rather
than the application.

This commit updates SpringBootServletInitializer to configure the
main class of the SpringApplication that it creates to be the
application's SpringBootServletInitializer subclass. This is done
prior to calling configure, so the main class can still be specified
by the application if required.

Closes gh-3061