Fixed #18430 -- Use the FILE_CHARSET setting when reading from a file during post processing with the cached staticfiles storage. Thanks to Brant Young for initial debugging.

tests/regressiontests/staticfiles_tests/apps/test/static/test/nonascii.css

@@ -0,0 +1,5 @@
+body {
+  background: url('window.png');
+}
+
+.snowman:before { content: "☃"; }