Get ?_trace=1 working with CSV and streaming CSVs

[simonw]

I think it's worth getting ?_trace=1 to work with streaming CSV - this would have helped me spot this issue a long time ago.

Originally posted by @simonw in #1349 (comment)

[simonw]

Since traces only work with text/html and JSON at the moment, the easiest way to do this will be to wrap generated CSV in a HTML page in a textarea if the user specified ?_trace=1:

datasette/datasette/tracer.py

Lines 125 to 134 in c5ae119

	if "text/html" in content_type and b"</body>" in accumulated_body:
	extra = json.dumps(trace_info, indent=2)
	extra_html = f"<pre>{extra}</pre></body>".encode("utf8")
	accumulated_body = accumulated_body.replace(b"</body>", extra_html)
	elif "json" in content_type and accumulated_body.startswith(b"{"):
	data = json.loads(accumulated_body.decode("utf8"))
	if "_trace" not in data:
	data["_trace"] = trace_info
	accumulated_body = json.dumps(data).encode("utf8")
	await send({"type": "http.response.body", "body": accumulated_body})