Provide default User-Agent in SimpleAsyncHTTPClient

[yan12125]

From https://tools.ietf.org/html/rfc7231:

A user agent SHOULD send a User-Agent field in each request unless specifically configured not to do so.

CurlAsyncHTTPClient provides default User-Agent string:

tornado/tornado/curl_httpclient.py

Lines 380 to 383 in c92b883

	if request.user_agent:
	curl.setopt(pycurl.USERAGENT, native_str(request.user_agent))
	else:
	curl.setopt(pycurl.USERAGENT, "Mozilla/5.0 (compatible; pycurl)")

While SimpleAsyncHTTPClient writes User-Agent only when it's provided by the user:

tornado/tornado/simple_httpclient.py

Lines 393 to 396 in c92b883

	if self.request.user_agent:
	self.request.headers["User-Agent"] = self.request.user_agent
	if not self.request.allow_nonstandard_methods:
	# Some HTTP methods nearly always have bodies while others

Could you consider adding default User-Agent string in SimpleAsyncHTTPClient?

[piraz]

What should be the default user agent to be added here? "Mozilla/5.0 (compatible; tornado)" is enough?

[ploxiln]

I think that is reasonable. More common for non-browsers, though, is something like: "Tornado/6.0"

(the curl command-line tool uses "curl/7.67.0", python3 urllib.request.urlopen uses "Python-urllib/3.7")

[piraz]

How about that @ploxiln?

[ploxiln]

Sorry, I should have pointed to #2749 where there is already commentary from the maintainer on this (but no response from the author, maybe your PR can replace it)

[piraz]

The PR #2749 will break the build because it isn't checking headers previously set. This is fixed with this PR.