Fix invalid call to value method with nil pointer

Now checking if a reflected value is invalid and, if so, making sure to
initialize the reflected value with the type's expected zero-value
before attempting to call the interface method.

This prevents panics due to invocation through reflection, such as:
"value method query.EncodedArgs.EncodeValues called using nil
*EncodedArgs pointer"

Fixes #9

query/encode.go

diff --git a/query/encode.go b/query/encode.go
index 396d804..90dcabb 100644
--- a/query/encode.go
+++ b/query/encode.go
@@ -167,6 +167,10 @@
 		}
 
 		if sv.Type().Implements(encoderType) {
+			if !reflect.Indirect(sv).IsValid() {
+				sv = reflect.New(sv.Type().Elem())
+			}
+
 			m := sv.Interface().(Encoder)
 			if err := m.EncodeValues(name, &values); err != nil {
 				return err

query/encode_test.go

diff --git a/query/encode_test.go b/query/encode_test.go
index 09efb0f..e0b2a36 100644
--- a/query/encode_test.go
+++ b/query/encode_test.go
@@ -265,6 +265,21 @@
 	}
 }
 
+func TestValues_MarshalerWithNilPointer(t *testing.T) {
+	s := struct {
+		Args *EncodedArgs `url:"arg"`
+	}{}
+	v, err := Values(s)
+	if err != nil {
+		t.Errorf("Values(%q) returned error: %v", s, err)
+	}
+
+	want := url.Values{}
+	if !reflect.DeepEqual(want, v) {
+		t.Errorf("Values(%q) returned %v, want %v", s, v, want)
+	}
+}
+
 func TestTagParsing(t *testing.T) {
 	name, opts := parseTag("field,foobar,foo")
 	if name != "field" {