Update core.js

https://github.com/andreareginato/simple-oauth2/issues/14

Adding more config options for https post, with properties like 
`ca`
`rejectUnauthorized`
`agent`
Update core.js

Checking if rejectUnauthorized is defined or not, possible value for rejectUnauthorized is true or false.
Merge pull request #15 from tomalex0/master

Update core.js