Fix replacing of headers with overlapping names. Fixes issue #3483 (#3487)

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.

Author: Jeroen88

Diff for: libraries/HTTPClient/src/HTTPClient.cpp

@@ -910,7 +910,7 @@ void HTTPClient::addHeader(const String& name, const String& value, bool first,
 
         if (replace) {
             int headerStart = _headers.indexOf(headerLine);
-            if (headerStart != -1) {
+            if (headerStart != -1 && (headerStart == 0 || _headers[headerStart - 1] == '\n')) {
                 int headerEnd = _headers.indexOf('\n', headerStart);
                 _headers = _headers.substring(0, headerStart) + _headers.substring(headerEnd + 1);
             }
@@ -924,7 +924,6 @@ void HTTPClient::addHeader(const String& name, const String& value, bool first,
             _headers += headerLine;
         }
     }
-
 }
 
 void HTTPClient::collectHeaders(const char* headerKeys[], const size_t headerKeysCount)