Make DefaultClient a method that returns the global default client

I made this a method to prevent other users from reassigning the
previously exported DefaultClient, e.g. doing something like:

```go
reddit.DefaultClient = nil
```

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

reddit/reddit_test.go

@@ -134,7 +134,7 @@ func TestNewReadonlyClient_Error(t *testing.T) {
 }
 
 func TestDefaultClient(t *testing.T) {
-	require.NotNil(t, DefaultClient)
+	require.NotNil(t, DefaultClient())
 }
 
 func TestClient_Readonly_NewRequest(t *testing.T) {