Fix interaction between unread and mark

[nhz2]

On master

julia> using TranscodingStreams

julia> stream = NoopStream(IOBuffer("foobar"));

julia> mark(stream)
0

julia> read(stream, String)
"foobar"

julia> TranscodingStreams.unread(stream, b"bar")

julia> reset(stream)
-3

julia> read(stream, String)
"foobarbar"

With this PR:

julia> using TranscodingStreams

julia> stream = NoopStream(IOBuffer("foobar"));

julia> mark(stream)
0

julia> read(stream, String)
"foobar"

julia> TranscodingStreams.unread(stream, b"bar")

julia> reset(stream)
0

julia> read(stream, String)
"foobar"

[nhz2]

These changes ensure the marked data and buffered data stay together.

[mkitti]

Looks food. I'm not sure if we should be using @Assert so much