Allow repeated keys in query params

[aaronblenkush]

This will allow repeated params in the query string as requested in issue #13.

query->map

(= (query->map "fq=1&fq=2&a=4&fq=3")
    {"fq" ["1" "2" "3"], "a" "4"})

map->query

(= (map->query {"fq" ["1" "2" "3"], "a" "4"})
    "a=4&fq=1&fq=2&fq=3")

[LukasRychtecky]

Could it be merged?

[saintx]

Let's get this merged! Looks great to me.

[saintx]

My only critique is the lack of tests.