Partial revert of 255a39cf5bc5ed76342. Overriding content type like it was
done here won't work because we can't allow content types that don't
specify a charset (we need a Reader for the output stream). Plus, such an
override is useless if the stylesheet defines its own xsl:output
directive.
Do not enforce HTML content type on output. Let the XSLT stylesheet decide what mime type should be used by explicit xsl:output directive.