SPDY: fixed off_t/size_t type conversions on 32 bits platforms.

SPDY: fixed off_t/size_t type conversions on 32 bits platforms.

Parameters of ngx_http_spdy_filter_get_shadow() are changed from size_t to off_t
since the last call of the function may get size and offset from the rest of a
file buffer.  This fixes possible data loss rightfully complained by MSVC on 32
bits systems where off_t is 8 bytes long while size_t is only 4 bytes.

The other two type casts are needed just to suppress warnings about possible
data loss also complained by MSVC but false positive in these cases.