errbot, or Slack, is URI encoding special characters like "greater than"

[lingfish]

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I am running...

• Errbot version: 5.2.0
• OS version: Debian Stretch
• Python version: 3.6.4
• Using a virtual environment: yes

Issue description

When using the configuration system, it seems that data being stored is being URI encoded by either the bot, or Slack (eg "greater than" > is being stored as ">".

Steps to reproduce

Setup a basic config dict, and then attempt to store the string "123>456".

When querying that back, it appears to be right in the Slack client, but when I use that field for say, a password, and send that to an external API, it sends > instead.

You can see this with a quick self.log call within plugin code.

[lingfish]

I just found #964 which seems to be related.

I think the only way around this until devs have a better idea, is for the user to have to supply (for example) a base64 encoded string, and unwrap it in the plugin.

As I think of that suggestion a little more, even the trailing equals may still get mangled :(