Initialize FrameworkServlet property sources eagerly

Prior to this change,
FrameworkServlet#configureAndRefreshWebApplicationContext called
 #postProcessWebApplicationContext(wac) and #applyInitializers(wac)
prior to #refresh, but because servlet-based property source stubs were
not replaced until #refresh, any post-processing or initialization
routines could not benefit from accessing the Environment to retrieve
properties from the ServletContext or ServletConfig.

The workaround to this problem is detailed in SPR-9610 - the user simply
needed to call WebApplicationContextUtils#initServletPropertySources
manually within their ApplicationContextInitializer (or overridden
 #postProcessWebApplicationContext method)

This commit ensures that
FrameworkServlet#configureAndRefreshWebApplicationContext calls
WebApplicationContextUtils#initServletPropertySources eagerly, prior to
invoking #postProcessWebApplicationContext and #applyInitializers.
Related Javadoc has also been updated throughout to clarify the behavior
of #initServletPropertySources, when it can be called and what the
effects are, etc.

Note also that a reproduction issue was added to demonstrate the problem
and verify its resolution [1].

[1]: https://github.com/SpringSource/spring-framework-issues/tree/master/SPR-9610

Issue: SPR-9610

spring-webmvc/src/main/java/org/springframework/web/servlet/FrameworkServlet.java

@@ -634,6 +634,12 @@ public abstract class FrameworkServlet extends HttpServletBean {
 		wac.setNamespace(getNamespace());
 		wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));
 
+		// the wac environment's #initPropertySources will be called in any case when
+		// the context is refreshed; do it eagerly here to ensure servlet property sources
+		// are in place for use in any post-processing or initialization that occurs
+		// below prior to #refresh
+		wac.getEnvironment().initPropertySources(getServletContext(), getServletConfig());
+
 		postProcessWebApplicationContext(wac);
 
 		applyInitializers(wac);