Don't clear headers on redirect (#5973)

Current implementation clears _headers when request was sent. If the
user added custom request headers, they will be lost and the redirected
request will not contain them. This commit changes the scope of cleanup
so that the headers survive redirects but don't survive connection
reuse.

libraries/HTTPClient/src/HTTPClient.cpp

@@ -200,6 +200,7 @@ bool HTTPClient::begin(String url, const char* CAcert)
         end();
     }
 
+    clear();
     _port = 443;
     if (!beginInternal(url, "https")) {
         return false;
@@ -226,6 +227,7 @@ bool HTTPClient::begin(String url)
         end();
     }
 
+    clear();
     _port = 80;
     if (!beginInternal(url, "http")) {
         return begin(url, (const char*)NULL);
@@ -243,7 +245,6 @@ bool HTTPClient::begin(String url)
 bool HTTPClient::beginInternal(String url, const char* expectedProtocol)
 {
     log_v("url: %s", url.c_str());
-    clear();
 
     // check for : (http: or https:
     int index = url.indexOf(':');
@@ -1212,8 +1213,8 @@ int HTTPClient::handleHeaderResponse()
         return HTTPC_ERROR_NOT_CONNECTED;
     }
 
-    clear();
-
+    _returnCode = 0;
+    _size = -1;
     _canReuse = _reuse;
 
     String transferEncoding;