Return errors on nil credentials, move code around

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

reddit/reddit-user-agent.go

@@ -0,0 +1,42 @@
+package reddit
+
+import "net/http"
+
+// cloneRequest returns a clone of the provided *http.Request.
+// The clone is a shallow copy of the struct and its Header map,
+// since we'll only be modify the headers.
+// Per the specification of http.RoundTripper, we should not directly modify a request.
+func cloneRequest(r *http.Request) *http.Request {
+	r2 := new(http.Request)
+	*r2 = *r
+	// deep copy of the Header
+	r2.Header = make(http.Header, len(r.Header))
+	for k, s := range r.Header {
+		r2.Header[k] = append([]string(nil), s...)
+	}
+	return r2
+}
+
+// Sets the User-Agent header for requests.
+type userAgentTransport struct {
+	userAgent string
+	Base      http.RoundTripper
+}
+
+func (t *userAgentTransport) setUserAgent(req *http.Request) *http.Request {
+	req2 := cloneRequest(req)
+	req2.Header.Set(headerUserAgent, t.userAgent)
+	return req2
+}
+
+func (t *userAgentTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	req2 := t.setUserAgent(req)
+	return t.base().RoundTrip(req2)
+}
+
+func (t *userAgentTransport) base() http.RoundTripper {
+	if t.Base != nil {
+		return t.Base
+	}
+	return http.DefaultTransport
+}