Generalize multiple field values to use Iterable.

This change allows retrofit to transform methods like:
```java
@POST
@FormUrlEncoded
void sendArray(@Field("order") List<String> array )
```
into: `order=xxx&order=xxx`

It's working for PHP arrays too, so we can use: `@Field("order[]") List<String> array`, to generate the expected php array in the http request: `order[]=xxx&order[]=xxx`

It add to `@Field` parameters a similar behaviour to we already have for query parameters  (https://github.com/square/retrofit/pull/380)