Fix params2list() duplicate key behavior

[ike709]

At some point BYOND changed duplicate key behavior from "use the last value" to "make a list of all the values".

From the ref:

If you have multiple items with the same name, they will be combined into a list of text strings. For example, "key=value1;key=value2" would set list["key"] to a list containing "value1" and "value2", not necessarily in that order.

See also #2444 and #2427. This ticks a box on the latter.