spdycat: cannot read post data from /dev/stdin

Reproduce:
  $ echo hello | spdycat -2 -v -d - https://localhost:443/test_post
  ...
  [  0.006] send RST_STREAM frame <version=2, flags=0, length=8>
            (stream_id=1, status_code=6)
  ...

  Error occurs when spdycat reads post data from /dev/stdin, in which case
  spdycat will send RST_STREAM(status_code=6(internal error)) to server.

Reason:
  In spdycat.cc, file_read_callback() use pread() to read post data.
  If post data file is /dev/stdin (pipe, socket or FIFO), pread() will
  returns -1 and set errno to ESPIPE.
  pread() is based on lseek(), but lseek() cannot works with pipe,
  socket or FIFO file.