oauth2: Add support for custom params in Exchange

Allows implementation of PKCE https://www.oauth.com/oauth2-servers/pkce/
for secure code exchange.

Fixes golang/oauth2#286

Change-Id: I07b6fefe6834a2406e8ca2fcbf118d38fc4875d1
GitHub-Last-Rev: 31c5ccbed38ba4998008180805be91adf480b859
GitHub-Pull-Request: golang/oauth2#285
Reviewed-on: https://go-review.googlesource.com/111515
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

oauth2_test.go

@@ -135,6 +135,52 @@ func TestExchangeRequest(t *testing.T) {
 	}
 }
 
+func TestExchangeRequest_CustomParam(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.String() != "/token" {
+			t.Errorf("Unexpected exchange request URL, %v is found.", r.URL)
+		}
+		headerAuth := r.Header.Get("Authorization")
+		if headerAuth != "Basic Q0xJRU5UX0lEOkNMSUVOVF9TRUNSRVQ=" {
+			t.Errorf("Unexpected authorization header, %v is found.", headerAuth)
+		}
+		headerContentType := r.Header.Get("Content-Type")
+		if headerContentType != "application/x-www-form-urlencoded" {
+			t.Errorf("Unexpected Content-Type header, %v is found.", headerContentType)
+		}
+		body, err := ioutil.ReadAll(r.Body)
+		if err != nil {
+			t.Errorf("Failed reading request body: %s.", err)
+		}
+		if string(body) != "code=exchange-code&foo=bar&grant_type=authorization_code&redirect_uri=REDIRECT_URL" {
+			t.Errorf("Unexpected exchange payload, %v is found.", string(body))
+		}
+		w.Header().Set("Content-Type", "application/x-www-form-urlencoded")
+		w.Write([]byte("access_token=90d64460d14870c08c81352a05dedd3465940a7c&scope=user&token_type=bearer"))
+	}))
+	defer ts.Close()
+	conf := newConf(ts.URL)
+
+	param := SetAuthURLParam("foo", "bar")
+	tok, err := conf.Exchange(context.Background(), "exchange-code", param)
+	if err != nil {
+		t.Error(err)
+	}
+	if !tok.Valid() {
+		t.Fatalf("Token invalid. Got: %#v", tok)
+	}
+	if tok.AccessToken != "90d64460d14870c08c81352a05dedd3465940a7c" {
+		t.Errorf("Unexpected access token, %#v.", tok.AccessToken)
+	}
+	if tok.TokenType != "bearer" {
+		t.Errorf("Unexpected token type, %#v.", tok.TokenType)
+	}
+	scope := tok.Extra("scope")
+	if scope != "user" {
+		t.Errorf("Unexpected value for scope: %v", scope)
+	}
+}
+
 func TestExchangeRequest_JSONResponse(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.String() != "/token" {