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.servlet/src/test/java/org/springframework/web/context/ContextLoaderTests.java

@@ -61,6 +61,7 @@ import org.springframework.web.servlet.SimpleWebApplicationContext;
  * @author Sam Brannen
  * @author Chris Beams
  * @since 12.08.2003
+ * @see org.springframework.web.context.support.Spr8510Tests
  */
 public final class ContextLoaderTests {