Fix formatting error of estimated_hours

To fix following error on Redmine 4.1.1
```
NoMethodError (undefined method `floor' for "1.0":String):

lib/redmine/i18n.rb:90:in `format_hours'
plugins/redmine_messenger/app/models/messenger.rb:209:in `detail_to_field'
```

Use redmine's `to_hours` convention on estimated_hours
```
lib/redmine/core_ext/string/conversions.rb
29:        def to_hours

app/models/issue.rb
454:    write_attribute :estimated_hours, (h.is_a?(String) ? (h.to_hours || h) : h)
```

app/models/messenger.rb

@@ -206,7 +206,7 @@ class Messenger
       when 'tracker'
         value = object_field_value(Tracker, detail.value)
       when 'estimated_hours'
-        value = format_hours(value)
+        value = format_hours(value.is_a?(String) ? (value.to_hours || value) : value)
       when 'project'
         value = object_field_value(Project, detail.value)
       when 'status'