Returns true if this request's declared "Content-Type" matches the specified media/mime type
.
Specifically, this method matches the given type
against this request's "Content-Type" header.
req.is(type);
Assuming the request contains a "Content-Type" header, "text/html; charset=utf-8":
req.is('html');
// -> true
req.is('text/html');
// -> true
req.is('text/*');
// -> true