fix(Request): Unify behavior of get_param for lists

This patch changes get_param so that it behaves the same whether a
list was specified in the query string using the HTML form style (in
which each element is listed in a separate 'key=val' field) or in
the more compact API style (in which each element is comma-separated
and assigned to a single param instance, as in 'key=val1,val2,val3').

Closes #312