Make remaining function signature maps for get_headers consistent

[TysonAndre]

get_headers can have string keys, depending on $format.
Make sure that static analysis catches these mismatches first.

src/Phan/Language/Internal/FunctionSignatureMap_php71_delta.php
23:    'get_headers' => ['array|false', 'url'=>'string', 'format='=>'int', 'context='=>'resource'],
44:    'get_headers' => ['array|false', 'url'=>'string', 'format='=>'int'],
src/Phan/Language/Internal/FunctionSignatureMapReal.php
304:'get_headers' => 'array<int,array>|array<int,string>|false',

src/Phan/Language/Internal/FunctionSignatureMapReal_php73.php
299:'get_headers' => '?array<int,array>|?array<int,string>|?false',

Compare with:

src/Phan/Language/Internal/FunctionSignatureMap.php
3689:'get_headers' => ['array<int|string,array|string>|false', 'url'=>'string', 'format='=>'int', 'context='=>'resource'],
15694:'xdebug_get_headers' => ['array'],

See #1821