Fix an issue with persistent bad authData on failure to link a user.

[nlutsenko]

Fixes #467.

Used this piece of code to test:

- (void)testLinkWithStripsNewDataOnFail {
    id mock = PFStrictProtocolMock(@protocol(PFUserAuthenticationDelegate));
    [PFUser registerAuthenticationDelegate:mock forAuthType:@"a"];
    OCMStub([mock restoreAuthenticationWithAuthData:@{ @"a" : @"b" }]).andReturn(YES);

    PFUser *user = [PFUser user];
    user.username = @"yolo";
    user.password = @"yarr";
    XCTAssertTrue([user signUp:nil]);

    XCTestExpectation *linkExpectation = [self expectationWithDescription:@"link user"];
    [[user linkWithAuthTypeInBackground:@"a" authData:@{ @"a" : @"b" }] continueWithBlock:^id(BFTask<NSNumber *> *task) {
        XCTAssertTrue(task.faulted);
        XCTAssertNotNil(task.error);
        [linkExpectation fulfill];
        return nil;
    }];
    [self waitForTestExpectations];
    XCTAssertFalse([user isLinkedWithAuthType:@"a"]);

    [self simulateApplicationReboot];

    user = [PFUser currentUser];
    XCTAssertFalse([user isLinkedWithAuthType:@"a"]);
}