Use to_ascii_lowercase when checking for lnurl override start

When calling `instructions.to_lowercase().rfind("lnurl")` and then
using the resulting value as a slice index in `instructions`, we
implicitly assume that any unicode lowercasing will not change the
length of any characters. That's probably fine in practice, but it
seems like a weird assumption to make, so instead we swap it for
`to_ascii_lowercase`.

Author: TheBlueMatt

src/lib.rs

@@ -947,7 +947,7 @@ impl PaymentInstructions {
 					))
 				},
 			}
-		} else if let Some(idx) = instructions.to_lowercase().rfind("lnurl") {
+		} else if let Some(idx) = instructions.to_ascii_lowercase().rfind("lnurl") {
 			let mut lnurl_str = &instructions[idx..];
 			// first try to decode as a bech32-encoded lnurl, if that fails, try to drop a
 			// trailing `&` and decode again, this could a http query param