fix for signInProvider property of FIRAuthTokenResult returning nil (#3446)

* fix for signInProvider

* Add a test

* Use constant

Author: collinjackson

Example/Auth/Tests/FIRUserTests.m

@@ -1072,6 +1072,7 @@ - (void)testGetIDTokenResultSuccess {
       XCTAssertTrue(tokenResult.authDate && [tokenResult.authDate isKindOfClass:[NSDate class]]);
       XCTAssertTrue(tokenResult.expirationDate &&
           [tokenResult.expirationDate isKindOfClass:[NSDate class]]);
+      XCTAssertEqualObjects(tokenResult.signInProvider, FIREmailAuthProviderID);
       XCTAssertTrue(tokenResult.claims && [tokenResult.claims isKindOfClass:[NSDictionary class]]);
       [expectation fulfill];
     }];

Firebase/Auth/Source/User/FIRUser.m

@@ -938,7 +938,7 @@ - (nullable FIRAuthTokenResult *)parseIDToken:(NSString *)token error:(NSError *
                                 expirationDate:expDate
                                       authDate:authDate
                                   issuedAtDate:issuedDate
-                                signInProvider:tokenPayloadDictionary[@"sign_in_provider"]
+                                signInProvider:tokenPayloadDictionary[@"firebase"][@"sign_in_provider"]
                                         claims:tokenPayloadDictionary];
   return result;
 }