url-hexify values in key-value pairs

Fixes an issue found in #129 and is based on that branch, so that must be merged first.

When the comment body contains a `+` (or indeed, any other meta-character like `&` or `;`), errors would arise when posting.
This was introduced in 94046d6f392844a44053afc9e6ad3a71e38f011f. @Bruce-Connor – I'm assuming this just a typo?  This is the only line I'm really 'eh' about.