Don't reset COLUMN to NIL

This merge fixes issue #10 and adds a testsuite capturing issue #10 and passing when the issue is fixed.

Normally, flexi-streams reset `column` to `nil` when user performs a binary output, as there is no sensible way to update `column` after binary output. See [line 151](https://github.com/edicl/flexi-streams/blob/master/output.lisp#L151)

The problem in issue #10 is that `column` slot is reset to `nil` even if user performs a character output.

For example user calls `(cl:format flexi-stream <some-characters>)` which calls `(flex:stream-write-sequence flexi-stream <some-characters>)` which calls `(flex::write-sequence* flexi-stream <some-characters>)` which encodes characters to octets and calls `(flex:stream-write-sequence flexi-stream <byte-array>)`.

The last `flex:stream-write-sequence flexi-stream` sees binary array and thinks it is passed by user and resets `column` to `nil`.

The solution is to pass the octets array directly to the underlying binary stream.