fix: trim all whitespace when splitting &validate fields

[Ibochkarev]

A multi-line &validate chunk like:

&validate=`
name:required,
email:email,
phone:required,
message:required
`

silently skips validation on the last field. validateFields() split the chunk on , and cleaned each piece with trim(ltrim($v), ' '), which only strips leading whitespace and trailing spaces. The last piece keeps its trailing newline (from the line break before the closing backtick), so the parsed validator name becomes "required\n" instead of "required". method_exists() can't match that name, so the field falls through to the "validator not specified" branch and is treated as valid.

Reproduced with a plain PHP snippet against the explode()/trim() logic in validateFields(), confirming the trailing \n survives the old trim and is removed by trim($v).

A comment on the issue mentions a workaround (trailing comma after the last field) that used to add "an extra empty field" to validation in an older FormIt version. That side effect doesn't reproduce here: the !empty($key[0]) check right after the split already discards the empty piece a trailing comma produces, with or without this fix. Verified both the reported case and the trailing-comma workaround against the current validateFields() logic.

Fixes #276