If two headers with overlapping names are added while replace == true, like in:
```cpp
http.addHeader("api_token",  "pMXFOLpinQqajaRQJYMeWObg2XYmcX1");
http.addHeader("token", "1234");
```
then replacing went wrong. This is fixed with this PR.
		
	
If two headers with overlapping names are added while replace == true, like in:
```cpp
http.addHeader("api_token",  "pMXFOLpinQqajaRQJYMeWObg2XYmcX1");
http.addHeader("token", "1234");
```
then replacing went wrong. This is fixed with this PR.