oauth2: Resource Owner Password Credentials grant

Adds support for the Resource Owner Password Credentials grant type, which
allows trusted clients to exchange user credentials for an access token
directly. This is generally a bad idea, but is extremely useful in some
situations, where an external redirect is undesirable or impossible.

See https://tools.ietf.org/html/rfc6749#section-4.3

Change-Id: I28efd77957bcf8e1174e93ba0c64a990b94eb839
Reviewed-on: https://go-review.googlesource.com/3862
Reviewed-by: Burcu Dogan <jbd@google.com>

oauth2_test.go

@@ -210,6 +210,53 @@ func TestExchangeRequest_NonBasicAuth(t *testing.T) {
 	conf.Exchange(ctx, "code")
 }
 
+func TestPasswordCredentialsTokenRequest(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		defer r.Body.Close()
+		expected := "/token"
+		if r.URL.String() != expected {
+			t.Errorf("URL = %q; want %q", r.URL, expected)
+		}
+		headerAuth := r.Header.Get("Authorization")
+		expected = "Basic Q0xJRU5UX0lEOkNMSUVOVF9TRUNSRVQ="
+		if headerAuth != expected {
+			t.Errorf("Authorization header = %q; want %q", headerAuth, expected)
+		}
+		headerContentType := r.Header.Get("Content-Type")
+		expected = "application/x-www-form-urlencoded"
+		if headerContentType != expected {
+			t.Errorf("Content-Type header = %q; want %q", headerContentType, expected)
+		}
+		body, err := ioutil.ReadAll(r.Body)
+		if err != nil {
+			t.Errorf("Failed reading request body: %s.", err)
+		}
+		expected = "client_id=CLIENT_ID&grant_type=password&password=password1&scope=scope1+scope2&username=user1"
+		if string(body) != expected {
+			t.Errorf("res.Body = %q; want %q", string(body), expected)
+		}
+		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)
+	tok, err := conf.PasswordCredentialsToken(NoContext, "user1", "password1")
+	if err != nil {
+		t.Error(err)
+	}
+	if !tok.Valid() {
+		t.Fatalf("Token invalid. Got: %#v", tok)
+	}
+	expected := "90d64460d14870c08c81352a05dedd3465940a7c"
+	if tok.AccessToken != expected {
+		t.Errorf("AccessToken = %q; want %q", tok.AccessToken, expected)
+	}
+	expected = "bearer"
+	if tok.TokenType != expected {
+		t.Errorf("TokenType = %q; want %q", tok.TokenType, expected)
+	}
+}
+
 func TestTokenRefreshRequest(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.String() == "/somethingelse" {