fix: #1335: async middleware called twice

[mleguen]

Closes #1335

Async middleware where called:

• once for the innermost subcommand (expected)
• then once for every parent of this subcommand (unexpected)

Due to an incorrect check in the first run of middleware (before validation).

CI will probably fail as #1411 introduced a broken test in master.

[mleguen]

Woops! Forgot to add a test to prove the issue is fixed, coming soon...

[mleguen]

Ready for review!

[bcoe]

@mleguen you're bitten by Node 6 not having async yet, switching to a promise should do the trick.

[mleguen]

Thanks @bcoe

I knew it... :-)

It should be OK now.

[bcoe]

@mleguen released in yargs@14.2.0.