ESP8266HTTPClient sets Content-Length even on empty GET-requests

### Basic Infos

- [x] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [x] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [x] I have tested that the issue is present in current master branch (aka latest git).
- [x] I have searched the issue tracker for a similar issue.
- [x] If there is a stack dump, I have decoded it.
- [x] I have filled out all fields below.

#### Platform

- Hardware: ESP8266
- Core Version:
- Development Env: Any
- Operating System: Any

### Settings in IDE

### Problem Description

Due to [this line](https://github.com/esp8266/Arduino/blob/master/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp#L463), the HTTP client always sets the `Content-Length` header.
I think this header should be omitted for GET-requests with a length of 0.