Fixes large payload issues for sendRequest

[ramirocarra]

sendRequest has a major problem when sending a big payload, the comparator in the IF loop has its two operators changed at the same time, so the last part of payload is never sent. This is seen when you try to send a payload that needs more than one operation.

The modification has been tested for a 1500 bytes payload.

edit from maintainer: fixes #7049

[d-a-v]

Approving (even if it could be more simple by just commenting size -= written like suggested in #7049)

[ramirocarra]

commenting size -= written may work, but that will make the "amount to write" wrong for the last chunk, in line:

int towrite = std::min((int)size, (int)HTTP_TCP_BUFFER_SIZE);

It may take some random info from payload pointer

[d-a-v]

That is very true. But HTTP_TCP_BUFFER_SIZE is totally useless and we should have
towrite = size - byteswritten there. This is the kind of code that is coded and recoded many times through the core than can be simplified by one transfer function for all.

(I'm still approving your PR, other maintainers will comment)

[ramirocarra]

That´s right too. I haven´t ckeck the code in function _client->write() but i guess it will take only the amount of info the buffer size allows (keeping HTTP_TCP_BUFFER_SIZE is an unnecessary double verification?)

[d-a-v]

keeping HTTP_TCP_BUFFER_SIZE is an unnecessary double verification?

It's more of a History/Legacy leftover.

[devyte]

This will likely get simplified with #6979 , but merging for the immediate need.

[TD-er]

Is it possible this also may fix an issue I was having with running out of memory after a number of failed connection attempts to an MQTT broker?

[d-a-v]

This is unrelated because pubsubclient does not use HTTPClient.

[TD-er]

OK, thanks, that does limit my search to the real problem that may have been fixed now :)