ensure gson does not attempt to decode void

gson/src/main/java/feign/gson/GsonModule.java

@@ -103,7 +103,7 @@ public final class GsonModule {
     }
 
     @Override public Object decode(Response response, Type type) throws IOException {
-      if (response.body() == null) {
+      if (void.class.equals(type) || response.body() == null) {
         return null;
       }
       Reader reader = response.body().asReader();

gson/src/test/java/feign/gson/GsonModuleTest.java

@@ -133,6 +133,22 @@ public class GsonModuleTest {
     }.getType()), zones);
   }
 
+  @Test public void voidDecodesToNull() throws Exception {
+    DecoderBindings bindings = new DecoderBindings();
+    ObjectGraph.create(bindings).inject(bindings);
+
+    Response response = Response.create(200, "OK", Collections.<String, Collection<String>>emptyMap(), zonesJson);
+    assertEquals(bindings.decoder.decode(response, void.class), null);
+  }
+
+  @Test public void nullBodyDecodesToNull() throws Exception {
+    DecoderBindings bindings = new DecoderBindings();
+    ObjectGraph.create(bindings).inject(bindings);
+
+    Response response = Response.create(204, "OK", Collections.<String, Collection<String>>emptyMap(), null);
+    assertEquals(bindings.decoder.decode(response, String.class), null);
+  }
+
   private String zonesJson = ""//
       + "[\n"//
       + "  {\n"//