Encode causality tokens using URL-safe b64

this should fix the pollitem test where the ct was passed in the url

src/model/k2v/causality.rs

@@ -42,11 +42,12 @@ impl CausalContext {
 			bytes.extend(u64::to_be_bytes(i));
 		}
 
-		base64::encode(bytes)
+		base64::encode_config(bytes, base64::URL_SAFE_NO_PAD)
 	}
 	/// Parse from base64-encoded binary representation
 	pub fn parse(s: &str) -> Result<Self, Error> {
-		let bytes = base64::decode(s).ok_or_message("Invalid causality token (bad base64)")?;
+		let bytes = base64::decode_config(s, base64::URL_SAFE_NO_PAD)
+			.ok_or_message("Invalid causality token (bad base64)")?;
 		if bytes.len() % 16 != 8 || bytes.len() < 8 {
 			return Err(Error::Message(
 				"Invalid causality token (bad length)".into(),