Server headers, FormData, some docs improvements and fix for bug in ChunkedResponse

[michalpokusa]

PR does not contain any breaking changes, I suggest publishing it as 4.1.0.

⭐ Added:

• FormData class and Request.form_data for parsing POST body for "application/x-www-form-urlencoded", "multipart/form-data", "text/plain" content types.
• Example that shows new functionality.
• Server.headers for setting headers that are sent with every response, without need for explicitly adding them in constructor.
• EDIT1: Server.poll() return value that indicates the result of polling e.g. NO_REQUEST or REQUEST_HANDLED_RESPONSE_SENT.

🛠️ Updated:

• Multiple docstrings are more verbose.
• Fix: In ChunkedResponse sending empty chunk was breaking functionality, because it ended response prematurely.
• Updated docs.
• EDIT1: Server.stopped is set to True in constructor, it changes to False only after calling .start() .server_forever().

🏗️ Refactor:

• Unified query_params and form_data to use _IFieldStorage for shared functionality.

[michalpokusa]

If possible, please test the impact of these changes to request processing speed. In theory there should not be much overhead, but in my testing I noticed a very big range (80ms-500ms). Although this might be caused by my WiFi and the connection itself, it we be great it anyone could eliminate the possibility that it is the code's fault.

Thanks in advance. 🙂