Library needs to interpret strings as SHIFT_JIS encoded strings for swf versions <6

Per SWF specification, *all* strings are encoded in SHIFT_JIS (which is a superset of ASCII) prior to version 6, However the library assumes all strings to be UTF-8 (which is the correct behaviour for >= version 6, however not for ones prior to that) which causes the reader/writer to produce invalid characters when non-ASCII characters are read.