Issue title: RFC4180Iterator and invalid field
 Issue body: ### Feature Request

|    Q        |   A
|------------ | ------
| New Feature | yes
| BC Break    | no

#### Proposal

the new RFC4180Iterator which is a userland CSV parser that try to conform to the informal RFC4180 spec. has landed on the master branch see issue #309 . But a last issue has pop up in how it should handle invalid record field. An invalid field is:

- a field containing the enclosure character while not being enclosed.
- a field containing the enclosure character not double while being enclosed.

Currently in both case the parser goes through without issuing any warning. 

Should the parser silently continue this behavior (and thus, we should clearly document it) or should it trigger an Exception instead ?  @theodorejb  what do you think ?  

 Comments: 
 Comment 0: We could keep the current parser behavior if we follow the Robustness principle, 

>  Be liberal in what you accept, and strict in what you produce 

As long as we document this behavior. What do you think ?
 Comment 1: Does `fgetcsv` also support reading those invalid fields without warnings? If so, I think the custom parser should continue supporting it, so that files readable with the native function will also be readable via the custom parser. And in general I think it's good to follow the Robustness Principle.
 Comment 2: Yes `fgetcsv` supports this but the resulting field content is different. I'd say that the custom parser results are more predicable than `fgetcsv` results 

```php
<?php

use League\Csv\Reader;

$csv = Reader38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5reateFromString('"foo"bar",foo"bar');
var_dump(iterator_to_array($csv));
// [['foobar"', 'foo"bar']]
$csv->setEscape('');
var_dump(iterator_to_array($csv));
// [['foo"bar', 'foo"bar']]
```

 Comment 3: After some digging I've managed to reproduce `fgetcsv` behavior using my code so once I've commit my changes this issue will be resolved.
{'source': 'GithubIssues', 'repo_name_with_owner': 'thephpleague/csv', 'issue_id': 362832195, 'repo_stars': 3013.0}