Diff sections are no longer decoded to Unicode strings if an encoding is

specified. Instead, they're always represented as byte strings. This is

more consistent for consumers to work with, and gives them the choice

as to whether they want to use the byte string as-is (such as when

writing to disk or a stream) or whether they want to decode it

themselves.
On the writing end, encodings were still being inherited, which could

end up causing the wrong encoding to be set. Now, diff sections are set

to not inherit the encodings of a parent section.

Unit tests pass.