SPDY: fixed operator precedence in uint16/uint32 write macros.

Since the type cast has precedence higher than the bit shift operator,
all values were truncated to 8 bits.

These macros are used to construct header block for SYN_REPLY frame on
platforms with strict alignment requirements.  As a result, any response
that contains a header with name or value longer than 255 bytes was
corrupted on such platforms.