Bug: Custom HTTP headers are lost on redirect in HTTPClient

[ZakKemble]

Custom HTTP headers (using HTTPClient::addHeader()) are lost if a redirect is encountered during the request. This fix (espressif/arduino-esp32#5973) needs to be applied here.

This bug is very troublesome when your API key is sent in the headers D:

[earlephilhower]

@ZakKemble you have a bug, a failing test case, and some code changes that fix it. Would you like to make a PR, since you have the best setup to test that it actually works?