Fixed wrong encoding of post parameters with tomcat

I want to use Ninja web framework 3.0.0 with Apache Tomcat 7.0.50.
But I found a problem: HTML POST parameters are not encoded by UTF-8.

Default encoding of application servers:

|               | GET                                       | POST       |
|---------------|-------------------------------------------|------------|
| Jetty 9.1.1   | UTF-8                                     | UTF-8      |
| Tomcat 7.0.50 | ISO-8859-1 or UTF-8(URIEncoding="UTF-8") | ISO-8859-1 |

I think that ```HttpServletRequest#setCharacterEncoding("utf-8")``` should be invoked in any case.

I tested with below repository.
[mallowlabs/ninja-encoding-test](https://github.com/mallowlabs/ninja-encoding-test)