Add CORS support to custom headers in transaction's api

[rferro]

Make POST calls to transaction's api enable CORS with pre-flight because custom headers are required.

ExpressJS sends by default:

Access-Control-Allow-Headers: Origin, X-Requested-With, Content-Type, Accept
Access-Control-Allow-Origin: *

Without custom headers, everything works.

Adding express cors to load in the server, or load only in the post routes that require custom headers, everything will work perfectly.