Consider missing/empty agent to be plaintext

[DavidBuchanan314]

This PR adds "" (empty string) to the PLAIN_TEXT_AGENTS list.

This means that a client that doesn't set the user-agent header at all (or sets it to an empty string) will be treated as plaintext.

Rationale: A client that doesn't set the UA is likely to be bare-bones, and therefore likely plaintext.

[chubin]

@DavidBuchanan314 It will not work like this because of how the agent check is implemented:

html_output = not any(agent in user_agent for agent in PLAIN_TEXT_AGENTS)

It does not check if the string is fully equal to the agent name, but only if the agent name is contained in the user agent header.

Why exactly do you need an empty agent check?

If you absolutely need it, feel free to fix this check:

https://github.com/chubin/wttr.in/blob/master/lib/wttr_srv.py#L166

For example, like this:

agent and not any(agent in user_agent for agent in PLAIN_TEXT_AGENTS)

[chubin]

(see above ^^^^^^)

[DavidBuchanan314]

any(agent in user_agent for agent in PLAIN_TEXT_AGENTS)

would evaluate to True if user_agent is empty string and PLAIN_TEXT_AGENTS contains empty-string, hence my suggestion.

Your suggestion to edit the expression would work too, though. (I just though it'd be cleaner to have a "data only" change vs a code change but it doesn't really matter either way).

As for why specifically I wanted it, I wrote a code-golfed curl-like program, and thought wttr.in would make a cool demo for it (https://github.com/DavidBuchanan314/kurl) - it does not support setting a UA header.