Default Content-Type set to text/html using setContentType, also UTF-8

See description in issue #91 (https://github.com/perwendel/spark/issues/91)
Rather than recycling Accept request field as Content-Type in response, it would be better to default to text/html. Accept field is often not appropriate, especially not for Internet Explorer.
Also better to use setContentType rather than setHeader as Tomcat does not allow Content-Type to be set using the latter method.