…taPart (jvahldick)

This PR was merged into the 4.4 branch.

Discussion
----------

[Mime] Fixing multidimensional array structure with FormDataPart

| Q             | A
| ------------- | ---
| Branch?       | 4.3
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #33063 #34031
| License       | MIT
| Doc PR        | -

The issue is pretty much described on #34031
The current structure of the raw body build on FormDataPart is not well recognized by the server. It considers all the fields as a root type, when actually it is possible to send arrays by html forms.

Lets the following structure on the html
```html
<input type="text" name="names[]" value="John" />
<input type="text" name="names[]" value="Doe" />
```

It creates the following raw body:
```
----------------------------466490401959219490193856
Content-Disposition: form-data; name="names[]"

John
----------------------------466490401959219490193856
Content-Disposition: form-data; name="names[]"

Doe
----------------------------466490401959219490193856--
```

Meanwhile, the FormDataPart on Mime component generates the following body:
```
--_=_symfony_1571410799_b7846b3b4e86d821cdec4379e62b4068_=_
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Content-Disposition: form-data

John
--_=_symfony_1571410799_b7846b3b4e86d821cdec4379e62b4068_=_
Content-Type: text/plain; charset=utf-8; name=1
Content-Transfer-Encoding: 8bit
Content-Disposition: form-data; name=1

Doe
--_=_symfony_1571410799_b7846b3b4e86d821cdec4379e62b4068_=_--
```

For more complex structures, the $_POST doesn't even recognize properly the field names and values.

Commits
-------

ca630e5 Changing the multipart form-data behavior to use the form name as an array, which makes it recognizable as an array by PHP on the $_POST globals once it is coming from the HttpClient component