Support Iterable and Array on query and form url encoded parameters

This allows retrofit to send query and form parameters that are represented as an Array or a Iterable on Java.

For example:

```java
String[] someParams = {"value1", "value2"};
```

Will get serialized as
```
paramName=value1&paramName=value2
```

<i>It is important to note that this is not an official standard, but it is used by many frameworks to pass array parameters</i>