BUG/MINOR: cache: Inverted variables in http_calc_maxage function

The maxage and smaxage variables were inadvertently assigned the
Cache-Control s-maxage and max-age values respectively when it should
have been the other way around.

This can be backported on all branches after 1.8 (included).
diff --git a/src/cache.c b/src/cache.c
index a73f266..f86c96f 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -495,7 +495,7 @@
 
 			chunk_strncat(chk, value, ctx.value.len - 8 + 1);
 			chunk_strncat(chk, "", 1);
-			maxage = atoi(chk->area);
+			smaxage = atoi(chk->area);
 		}
 
 		value = directive_value(ctx.value.ptr, ctx.value.len, "max-age", 7);
@@ -504,7 +504,7 @@
 
 			chunk_strncat(chk, value, ctx.value.len - 7 + 1);
 			chunk_strncat(chk, "", 1);
-			smaxage = atoi(chk->area);
+			maxage = atoi(chk->area);
 		}
 	}