Utf-8 and Content-Length header

A general problem exists in libwww when POSTing data to HTTP servers,
Content-Length headers and syswrite() calls use length() function over
payload to determine it's size.  This fails on Utf-8 encoded data,
because length() function returns the count of _characters_, not count
of bytes. It leads to a message truncated.

Solution is to use bytes::length() instead - it returns the size of
string in bytes.

Patch attached.

-- 
Pav Lucistnik <pav@xxxxxxx>
              <pav@xxxxxxxxxxx>

Every man is as God made him, ay, and often worse.
  -- Miguel de Cervantes

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