Preserve programmatically set context config locations

Prior to this fix, ContextLoader(Listener)'s would overwrite any
value set directly against a WebApplicationContext's #setConfigLocation
method. This is a likely scenario when using Spring 3.1's new
WebApplicationInitializer support.

Now a check is performed to ensure that the ContextLoader init-param
value is non-null before doing the overwriting.

Added tests to ensure that all expected precedence, overwriting and
defaulting of context config locations works as expected.

Issue: SPR-8510

org.springframework.web/src/main/java/org/springframework/web/context/ContextLoader.java

@@ -376,7 +376,10 @@ public class ContextLoader {
 
 		wac.setParent(parent);
 		wac.setServletContext(sc);
-		wac.setConfigLocation(sc.getInitParameter(CONFIG_LOCATION_PARAM));
+		String initParameter = sc.getInitParameter(CONFIG_LOCATION_PARAM);
+		if (initParameter != null) {
+			wac.setConfigLocation(initParameter);
+		}
 		customizeContext(sc, wac);
 		wac.refresh();
 	}