login_hint should not be deleted from SAML AuthURIs

### [REQUIRED] Describe your environment

* Operating System version: Windows 10
* Browser version: Firefox 80.0.1 64-bit
* Firebase SDK version: 4.5.2
* Firebase Product: auth

### [REQUIRED] login_hint not appended to SAML AuthURI

#### Steps to reproduce:

Using any SAML provider, supply a login_hint (such as foo@example.com). Note that &login_hint=foo@example.com is not appended to the SAMLRequest URL

#### Relevant Code:

This test is wrong: https://github.com/firebase/firebase-js-sdk/blob/90b5f044b136211a39f891c05788bc46de1d2927/packages/auth/test/rpchandler_test.js#L7984
It is not a violation of the SAML standard to pass along custom parameters

This code is the root cause: https://github.com/firebase/firebase-js-sdk/blob/b73ab41356f5b86a693a0e6c5cdc1a6e9841b8e6/packages/auth/src/rpchandler.js#L1006