proxy: fix group impersonation bug

Signed-off-by: Bobby DeSimone <bobbydesimone@gmail.com>

docs/docs/CHANGELOG.md

@@ -23,6 +23,10 @@
 
 - Removed `AUTHENTICATE_INTERNAL_URL`/`authenticate_internal_url` which is no longer used.
 
+## Fixed
+
+- Fixed a bug where the impersonate form would persist an empty string for groups value if none set.[GH-303](https://github.com/pomerium/pomerium/issues/303)
+
 ## v0.3.0
 
 ### New

proxy/handlers.go

@@ -233,7 +233,10 @@ func (p *Proxy) Impersonate(w http.ResponseWriter, r *http.Request) {
 	// OK to impersonation
 	session.ImpersonateEmail = r.FormValue("email")
 	session.ImpersonateGroups = strings.Split(r.FormValue("group"), ",")
-
+	groups := r.FormValue("group")
+	if groups != "" {
+		session.ImpersonateGroups = strings.Split(groups, ",")
+	}
 	if err := p.sessionStore.SaveSession(w, r, session); err != nil {
 		httputil.ErrorResponse(w, r, err)
 		return