Preserve query when building redirect (fix for #695) (#696)

* Add test for GetRedirect to check query and fragments.

* Preserve query and fragment when building redirect.

* Add changelog entry for redirect fix

oauthproxy_test.go

@@ -1678,6 +1678,11 @@ func TestGetRedirect(t *testing.T) {
 			url:              "/foo/bar",
 			expectedRedirect: "/foo/bar",
 		},
+		{
+			name:             "request with query preserves query",
+			url:              "/foo?bar",
+			expectedRedirect: "/foo?bar",
+		},
 		{
 			name:             "request under ProxyPrefix redirects to root",
 			url:              proxy.ProxyPrefix + "/foo/bar",