increase HTTP receive buffer size is to better handle rest API

REST API is supported in MongoDB.
However, the buffer size for HTTP protocol is secured only 4096 bytes in stack. ( MiniWebServer::accepted() in miniwebserver.cpp )
It seems like too small to handle POST method request that was intended in put a document into the DB.

For now , I could get the "400 OK" status code when I try to POST a document is exceeded 4k.