MINOR: compression: Rename the function check_legacy_http_comp_flt()

To not mix it up with the legacy HTTP representation, this function has been
rename check_implicit_http_comp_flt().
diff --git a/src/filters.c b/src/filters.c
index 6368b20..d082261 100644
--- a/src/filters.c
+++ b/src/filters.c
@@ -336,7 +336,7 @@
 		if (fconf->ops->check)
 			err += fconf->ops->check(proxy, fconf);
 	}
-	err += check_legacy_http_comp_flt(proxy);
+	err += check_implicit_http_comp_flt(proxy);
 	return err;
 }