Fix signature verification when request contains repeating params

For example, params like 

```clj
{:a "1" :b ["2" "3"]}
```

previously would produce:

```clj
"a=1&b=%5B%222%22%20%223%22%5D"     ; "a=1&b=["2" "3"]"
```

but now produce:

```clj
"a=1&b=2&b=3"
```