Fix #1725: Force URLSession's to use main thread for completion block

[Brandon-T]

Force the sessions to be on the main queue. This is what Alamofire does automatically.

Summary of Changes

This pull request fixes issue #

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue is assigned to a milestone (should happen at merge time).