Narrow parameter type for antispambot()

[IanDelMar]

Narrows $hex_encoding from int to 0|1 for antispambot().

While passing integers other than 0 (default) or 1 does not trigger PHP or WordPress errors, it expands the random range rand(0, 1 + $hex_encoding), allowing values (e.g. 3) that hit no branch and therefore omit characters. -1 would preserve correct email addresses, but since the parameter description says:

Set to 1 to enable hex encoding.

it has been narrowed to 0|1.