PHP (7.1.16) was showing `Content-Type` and `Content-Length` with both their original name and preceded by `HTTP`:
```
CONTENT_TYPE
HTTP_CONTENT_TYPE
CONTENT_LENGTH
HTTP_CONTENT_LENGTH
```

This was causing the headers to be added as duplicates in the request_headers array. Removing the 'CONTENT' condition to prevent the duplicate addition.

Remove the `Expect` header which is [not handled](https://blog.pilif.me/2007/02/02/the-return-of-except-100-continue/) by many hosts (including the API gateway apparently).

Restore CONTENT headers- it seems that from some clients PHP doesn't have the duplicate headers. Consider adding an `array_unique` to clean header array

Wrap handling of Expect header in config variable.

Add option to suppress Expect: 100-continue header