Fix bug in proxy where HTTPS_PROXY became 'undefined'

[flotwig]

This is the bug causing the failing AppVeyor build, basically the agent was always trying to use a proxy set to string "undefined" when no proxy is defined, making the request fail catastrophically every time.

Appveyor build: https://ci.appveyor.com/project/cypress-io/cypress-test-example-repos/builds/23690466

The issue causing the actual crash was that socket.io was somehow ACK'ing with a circular reference because of the error:

Mon, 08 Apr 2019 19:51:45 GMT socket.io-parser decoded 210["backend:request","http:request",{"url":"https://jsonplaceholder.cypress.io/users","method":"GET","gzip":true,"timeout":30000,"followRedirect":true}] as {"type":2,"nsp":"/","id":10,"data":["backend:request","http:request",{"url":"https://jsonplaceholder.cypress.io/users","method":"GET","gzip":true,"timeout":30000,"followRedirect":true}]}
Mon, 08 Apr 2019 19:51:45 GMT socket.io:socket got packet {"type":2,"nsp":"/","id":10,"data":["backend:request","http:request",{"url":"https://jsonplaceholder.cypress.io/users","method":"GET","gzip":true,"timeout":30000,"followRedirect":true}]}
Mon, 08 Apr 2019 19:51:45 GMT socket.io:socket emitting event ["backend:request","http:request",{"url":"https://jsonplaceholder.cypress.io/users","method":"GET","gzip":true,"timeout":30000,"followRedirect":true}]
Mon, 08 Apr 2019 19:51:45 GMT socket.io:socket attaching ack callback to event
Mon, 08 Apr 2019 19:51:45 GMT socket.io:socket dispatching an event ["backend:request","http:request",{"url":"https://jsonplaceholder.cypress.io/users","method":"GET","gzip":true,"timeout":30000,"followRedirect":true},null]
.......some more Cypress logs here........
Mon, 08 Apr 2019 19:51:47 GMT socket.io:socket sending ack [Circular]
RangeError: Maximum call stack size exceeded

This is known to crash socket.io: socketio/socket.io-parser#80

I fixed the proxy issue where process.env.HTTPS_PROXY was becoming "undefined" and added a test, but I'm still unsure as to how this manifested into the circular ack bug we got here.