Bug: incorrect signature generation leads to "Invalid signature" with non-ASCII in names

[slaFFik]

If the customer name in HelpScout or FreeScout has special characters, like ã, ü, ö - both FreeScout and HelpScout will not display in their sidebars EDD data.

The reason is in this method: \EDD\HelpScout\Request::create_expected_signature().

json_encode() encodes non-ASCII characters, so signatures are not equal when checked later in \EDD\HelpScout\Request::signature_equals().

The fix:

	private function create_expected_signature() {
		return base64_encode( hash_hmac( 'sha1', json_encode( $this->data, JSON_UNESCAPED_UNICODE ), self::$secret_key, true ) );
	}

Note the new JSON_UNESCAPED_UNICODE.

I will submit the PR.