Encode timestamps as float instead of string values

Some JWT parsers like auth0/node-jsonwebtoken and jwt.io complain about timestamps formatted as a string.

src/Encoding/MicrosecondBasedDateConversion.php

@@ -8,6 +8,7 @@
 use Lcobucci\JWT\Token\RegisteredClaims;
 
 use function array_key_exists;
+use function floatval;
 
 final class MicrosecondBasedDateConversion implements ClaimsFormatter
 {
@@ -25,16 +26,11 @@ public function formatClaims(array $claims): array
         return $claims;
     }
 
-    /** @return int|string */
+    /** @return float|string */
     private function convertDate(DateTimeImmutable $date)
     {
-        $seconds      = $date->format('U');
-        $microseconds = $date->format('u');
+        $formattedTimestamp = $date->format('U.u');
 
-        if ((int) $microseconds === 0) {
-            return (int) $seconds;
-        }
-
-        return $seconds . '.' . $microseconds;
+        return floatval($formattedTimestamp);
     }
 }