Throw ISEs in MockHttpSession for invalid session

The Javadoc for several methods in HttpSession specifies that an
IllegalStateException must be thrown if the method is called on an
invalidated session; however, Spring's MockHttpSession did not implement
this behavior consistently prior to this commit.

This commit therefore ensures that the following methods in
MockHttpSession properly throw an IllegalStateException as defined in
the Servlet specification.

 - long getCreationTime()
 - long getLastAccessedTime()
 - Object getAttribute(String)
 - Object getValue(String)
 - Enumeration<String> getAttributeNames()
 - String[] getValueNames()
 - void setAttribute(String, Object)
 - void putValue(String , Object)
 - void removeAttribute(String)
 - void removeValue(String)
 - void invalidate()
 - boolean isNew()

Issue: SPR-7659