The streaming decoder previously selected an encoding only from a byte-order
mark or the UTF sniff, so its single-byte and CJK decode paths were latent. It
now reads the encoding from the XML declaration (the libxml2 behavior) when
there is no BOM, activating those paths over an incremental byte source. Add
the streaming form of ISO-2022-JP, carrying the stateful mode across reads
(extracted to its own file to stay under the type-body cap). A test asserts
streaming agrees with whole-buffer decode for a single-byte (ISO-8859-1), a
CJK (Shift_JIS), and an ISO-2022-JP document.
Part of #108, #105
Mihaela Mihaljevic
191d2245de