Support for multivalued params

RequestParams should not use Maps to store values since many applications need to send multivalued params.

param=foo&param=bar is totally needed in many cases. Web frameworks usually parse this querystring and return an array or collection of values. See for example:
http://docs.oracle.com/javaee/1.3/api/javax/servlet/ServletRequest.html#getParameterValues(java.lang.String)