De-duplicate code in GetLoginURL of in logingov provider

Also add unit test to ensure logingov specific logic is applied.

providers/logingov.go

@@ -225,19 +225,11 @@ func (p *LoginGovProvider) Redeem(ctx context.Context, redirectURL, code string)
 
 // GetLoginURL overrides GetLoginURL to add login.gov parameters
 func (p *LoginGovProvider) GetLoginURL(redirectURI, state string) string {
-	a := *p.LoginURL
+	a, params := DefaultGetLoginURL(p.ProviderData, redirectURI, state)
-	params, _ := url.ParseQuery(a.RawQuery)
+	if p.AcrValues == "" {
-	params.Set("redirect_uri", redirectURI)
+		acr := "http://idmanagement.gov/ns/assurance/loa/1"
-	params.Set("approval_prompt", p.ApprovalPrompt)
-	params.Add("scope", p.Scope)
-	params.Set("client_id", p.ClientID)
-	params.Set("response_type", "code")
-	params.Add("state", state)
-	acr := p.AcrValues
-	if acr == "" {
-		acr = "http://idmanagement.gov/ns/assurance/loa/1"
-	}
 		params.Add("acr_values", acr)
+	}
 	params.Add("nonce", p.Nonce)
 	a.RawQuery = params.Encode()
 	return a.String()

providers/logingov_test.go

@@ -289,3 +289,10 @@ func TestLoginGovProviderBadNonce(t *testing.T) {
 	// The "badfakenonce" in the idtoken above should cause this to error out
 	assert.Error(t, err)
 }
+
+func TestLoginGovProviderGetLoginURL(t *testing.T) {
+	p, _, _ := newLoginGovProvider()
+	result := p.GetLoginURL("http://redirect/", "")
+	assert.Contains(t, result, "acr_values="+url.QueryEscape("http://idmanagement.gov/ns/assurance/loa/1"))
+	assert.Contains(t, result, "nonce=fakenonce")
+}