We say the header block is invalid if at least one of the following
condition is true:
There are duplicate header names; or the header names are not
encoded in US-ASCII character set and not lower cased; or the
header name is zero-length string; or the header value contains
multiple in-sequence NUL bytes.
spdylay_frame_unpack_nv() returns SPDYLAY_ERR_INVALID_HEADER_BLOCK
if the unpacking suceeded but it found the header block is invalid.
This means that caller treats it as success, but do additional
processing for invalid header block if it wants.
The functions calling spdylay_frame_unpack_nv() also return
SPDYLAY_ERR_INVALID_HEADER_BLOCK.
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