isAsyncFn should check if function filter returns a promise

[niftylettuce]

No description provided.

[niftylettuce]

example:

      curl: cmd => {
        cmd = cmd.split('\n').join('\\\n');
        return new Promise(async (resolve, reject) => {
          try {
            const response = await exec(cmd, {
              stdio: 'ignore',
              timeout: env.CURL_FILTER_TIMEOUT_MS
            });
            console.log('response', response);
            resolve(response.stdout ? response.stdout : response);
          } catch (err) {
            console.log(err && err.stack);
            resolve(err.stderr ? err.stderr : err.message);
          }
        });
      }

I have to rewrite this in order for it to return true for

koa-nunjucks-next/index.js

Line 17 in 2b8de10

const isAsyncFn = fn => {

[niftylettuce]

when it gets compiled down with babel this is removed I believe and therefore it has no support for this?

[beliefgp]

I have thought about your code when commit this fix.
it well run twice lead to affect performance when use return promise.
so I do not recommend using babel compilation，support node > 7.0