Allow for async host functions.

[monkpow]

@connorwhite huh, now these are broken out, I see we're missing test for these cases. I'll try to pick them up in the next day or so, or if you want to add tests I'll accept the commit then. Thanks!

[srinivasrk]

Hi, first off thanks for all your help. Is this PR merged into master? Is there no way to get async host resolution for now?

[devalexqt]

TypeError: Cannot read property 'request' of undefined
node_modules\express-http-proxy\app\steps\sendProxyRequest.js:13:51

[mitcns]

I met the same problem，TypeError, when I wanna use proxy to change req headers，this is the detail infos:
TypeError: Cannot read property 'request' of undefined
at /node_modules/_express-http-proxy@1.6.2@express-http-proxy/app/steps/sendProxyRequest.js:13:51
at new Promise ()
at sendProxyRequest (/node_modules/_express-http-proxy@1.6.2@express-http-proxy/app/steps/sendProxyRequest.js:11:10)
at process._tickCallback (internal/process/next_tick.js:68:7)

and check the console-term, it shows:
node_modules/send/index.js:1038
return typeof res.headersSent !== 'boolean'
^

TypeError: Cannot read property 'headersSent' of undefined