Summary:
Fix selection problem on ManageMockResponsePanel. Currently, when the user adds new routes, the detail section continues to show the detail for the first route. Select different routes in the list does not change the detail panel. The screen is currently not doing anything on selection.
Also fixed a problem with updating on mock header names and values. Updates were not persisting if user did not exist input fields before leaving header input panel.
## Changelog
Network Plugin - Fix selection problem on ManageMockResponsePanel
Pull Request resolved: https://github.com/facebook/flipper/pull/2574
Test Plan:
Add multiple mock routes
Select various routes
Verify that detail panel shows the selected route

Reviewed By: passy
Differential Revision: D29582286
Pulled By: mweststrate
fbshipit-source-id: 6690e2262a033cdfa5afa6e6fdfacc9694244590