Url encode requests from GSON objects :)

gistfile1.java

package uk.co.airsource.android.authtest;

import com.google.gson.Gson;
import com.google.gson.JsonElement;

import java.io.UnsupportedEncodingException;
import java.util.Map;
import java.util.Set;

import retrofit.converter.GsonConverter;
import retrofit.mime.FormUrlEncodedTypedOutput;
import retrofit.mime.TypedOutput;

/**
 * Created by sduke on 03/02/2015.
 */
public class GsonConverterUrlEncodedRequest extends GsonConverter
{
    private final Gson gson;

    private String charset;

    /**
     * Create an instance using the supplied {@link com.google.gson.Gson} object for conversion. Encoding to JSON and
     * decoding from JSON (when no charset is specified by a header) will use UTF-8.
     */
    public GsonConverterUrlEncodedRequest(Gson gson) {
        this(gson, "UTF-8");
    }

    /**
     * Create an instance using the supplied {@link Gson} object for conversion. Encoding to JSON and
     * decoding from JSON (when no charset is specified by a header) will use the specified charset.
     */
    public GsonConverterUrlEncodedRequest(Gson gson, String charset) {
        super(gson, charset);
        this.gson = gson;
        this.charset = charset;
    }

    @Override public TypedOutput toBody(Object object) {
        try {
            Set<Map.Entry<String, JsonElement>> fieldSet = gson.toJsonTree(object).getAsJsonObject().entrySet();
            FormUrlEncodedTypedOutput formUrlEncodedTypedOutput = new FormUrlEncodedTypedOutput();
            for (Map.Entry<String, JsonElement> entry : fieldSet)
            {
                if (!entry.getValue().isJsonPrimitive())
                {
                    throw new UnsupportedEncodingException(
                            "GSON to URL encoding only supports shallow (depth=1) objects. Key: "
                                    + entry.getKey() + " contains a non-primitive object!");
                }
                String valueString = entry.getValue().getAsJsonPrimitive().getAsString();
                formUrlEncodedTypedOutput.addField(entry.getKey(), valueString);
            }
            return formUrlEncodedTypedOutput;
        } catch (UnsupportedEncodingException | IllegalStateException e) {
            throw new AssertionError(e);
        }
    }
}

Thanks for useful tip!
Maybe it's better to serialize not primitives as Json instead of throw Exception:
String valueString = entry.getValue().toString();