Current SPDY/3 spec does not clearly prohibit to send
delta_window_size which makes resulting window size more than initial
window size. For this reason, spdylay_submit_window_update() can send
delta_window_size in [1, (1 << 31)-1], inclusive, without bounded by
stream's recv_window_size. Of course, the application is now
responsible to keep the resulting window size <= (1 << 31)-1.
spdylay_submit_window_update() now returns
SPDYLAY_ERR_INVALID_ARGUMENT if delta_window_size is 0 or negative.
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