fix: Serialize the full u64 range (#732)

This looks like something that was missed during #178.

Author: epage

src/ser.rs

@@ -121,15 +121,7 @@ impl<'a> ser::Serializer for &'a mut ConfigSerializer {
     }
 
     fn serialize_u64(self, v: u64) -> Result<Self::Ok> {
-        if v > (i64::MAX as u64) {
-            Err(ConfigError::Message(format!(
-                "value {} is greater than the max {}",
-                v,
-                i64::MAX
-            )))
-        } else {
-            self.serialize_i64(v as i64)
-        }
+        self.serialize_primitive(v)
     }
 
     fn serialize_f32(self, v: f32) -> Result<Self::Ok> {