improve tests: fix glitches re "null" vs "undefined"; consistent var names: a resource is always "res", the result of ".serialize(..)" is always "serialized"; "rejects" instead of "fails" where appropriate; simplified a few tests by using assert.content
redo resource-test.js/"Content-Type"; mark bogus test; 1 new test fails: utf-8 or base64 as default encoding of file with *empty extension*?

* test for content-type and content-type/charset *separately*
+ check .encoding for files with various extensions
+ tests for .HTM, .TXT and empty ext (".") as well as for several "binary" files;