Summary:
`XMLHttpRequest.getResponseHeader` is case-insensitive, therefor the React-Native implementation needs to mimic this behavior as to not break libraries that are dependent on this.
There is a corresponding issue in `superagent` but this is the root cause (https://github.com/visionmedia/superagent/issues/636).
Closes https://github.com/facebook/react-native/pull/1138
Github Author: Ryan Pastorelle <rpastorelle@yahoo.com>
Test Plan: Imported from GitHub, without a `Test Plan:` line.