v0.5.0

Abbreviate api.php URL as domain, warn on requests without user agent.

• BREAKING CHANGE (internal): The Session constructor now requires the default request options to include warn, which must be a function. The fetch.js and axios.js backends already add a default for this option, so this is only relevant for you if you wrote a custom network implementation; if you just import browser.js or node.js, it doesn’t matter.

• The first constructor argument can now be a domain name instead of a full api.php URL, e.g. en.wikipedia.org instead of
  https://en.wikipedia.org/w/api.php.

• Requests that do not specify a user agent will now trigger a warning, limited to once per session. If you see this warning, you should add a
  user agent to your requests – see the User-Agent policy. Usually you would add it to the default options at construction time:

  const session = new Session( 'en.wikipedia.org', {
      formatversion: 2,
      // other default params...
  }, {
      userAgent: 'my-cool-tool',
      // other default options...
  } );

  But it can also be specified for an individual request:

  const response = await session.request( {
      action: 'query',
      // other params...
  }, {
      userAgent: 'my-cool-tool',
      // other options...
  } );

  Recall that the default warning handler is console.warn in the browser, and also in Node.js if NODE_ENV = “development” is set, but otherwise the Node.js backend ignores warnings.