This should reduce the complexity of the function

This fixes the PHP Notice when $headers['Content-Type'] is unset. It
also allows for easier extension later, e.g. adding handling for a JSON
content-type.

This function might be called by an plugin with an array that has only
string keys and hence no key 0. This would trigger an PHP Notice when
trying to get the first key's level.

If an config value is not set in the input lines, initialize it as empty
string to prevent PHP Notices complaining about $line[1] being not set.

A PHP Notice occurred when $url didn't contain a '?'.

If SSL is not supported, a HTTP request will be made. In the log the request type will be indicated, but on the frontend no additional message would be shown (I think it's better to notify admins about non-SSL situations, but currently when this fetch encounter error, no error message will be shown, so it's better not to add any warning).

This makes it a little easier to splice new elements into position

While DokuWiki suppresses PHP Notices they are still a code smell and
should be fixed. This fixes some PHP Notices that occurred.

Some of these fixes could be refactored into nicer code once we move to
PHP 7 and get access to the `??` operator.

The HTML-spec[0] specifies:

> If the element has no label attribute and is not a child of a datalist element: Text that is not inter-element whitespace.

So a non-breaking space is valid solution.

[0] https://html.spec.whatwg.org/multipage/form-elements.html#the-option-element