http/hpack: fix integer encoding at the prefix maximum

[zeen]

Encoding an integer equal to the HPACK prefix maximum (2^N - 1) currently omits the required zero continuation byte. The decoder then consumes the next field's byte as part of the integer, potentially corrupting the header block
and causing an HTTP/2 connection error.

For example, encoding 31 with a five-bit prefix and mask 0 produces 1f instead of 1f 00. Decoding the current encoding followed by x yields 151 instead of 31.

RFC 7541 section 5.1 specifies that the single-byte representation applies only when I < 2^N - 1: https://www.rfc-editor.org/rfc/rfc7541.html#section-5.1

Minimal reproducer:

local hpack = require "http.hpack"
assert(hpack.encode_integer(31, 5, 0) == "\31\0")

[daurnimator]

Thanks! And great clean PR!