Keep sock original timeout, If read,write timeout not set

[iamdrq]

Summary

In httpcore/_backends/sync.py, when not set timeout. It will override original sock timeout setting,this patch fixed it.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.