[ZD#672511] fix FreshBooks app encoding error

:bug:

Rack assumes the request is x-www-form-urlencoded unless the Content-Type header is explicitly set:
https://github.com/rack/rack/blob/1.4.5/lib/rack/request.rb#L171

```
Unexpected error while processing request: invalid %-encoding ("1.0" encoding="UTF-8"?>
<request method="time_entry.create">
  <time_entry>
    <project_id>20130</project_id>
    <task_id>156576</task_id>
    <hours>1</hours>
    <notes><![CDATA[Ticket: 28 12%]]></notes>
    <staff_id>1</staff_id>
  </time_entry>
</request>)
    /opt/boxen/rbenv/versions/2.1.0/lib/ruby/2.1.0/uri/common.rb:901:in `decode_www_form_component'
```

#### References:

- [[ZD#672511] FreshBooks app errors out when a % is present in the notes field](https://support.zendesk.com/agent/#/tickets/672511)

/cc @maximeprades @liulikun