Chage NewRequest methods, add multi description kind to thing

Since Reddit's API accepts form data as the body for most of its
endpoints, it made sense to me to make the default NewRequest method
set the request body as form data (if provided of course). The
NewJSONRequest method can accept a JSON body.

Signed-off-by: Vartan Benohanian <vartanbeno@gmail.com>

reddit/multi_test.go

@@ -217,7 +217,7 @@ func TestMultiService_Delete(t *testing.T) {
 	require.NoError(t, err)
 }
 
-func TestMultiService_GetDescription(t *testing.T) {
+func TestMultiService_Description(t *testing.T) {
 	client, mux, teardown := setup()
 	defer teardown()
 
@@ -229,7 +229,7 @@ func TestMultiService_GetDescription(t *testing.T) {
 		fmt.Fprint(w, blob)
 	})
 
-	description, _, err := client.Multi.GetDescription(ctx, "user/testuser/m/testmulti")
+	description, _, err := client.Multi.Description(ctx, "user/testuser/m/testmulti")
 	require.NoError(t, err)
 	require.Equal(t, "hello world", description)
 }