Force responseHeaders to lower case to respect case-insensitivity

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 (ladjs/superagent#636).
Closes facebook#1138
Github Author: Ryan Pastorelle <rpastorelle@yahoo.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Network/XMLHttpRequestBase.js

@@ -66,7 +66,7 @@ class XMLHttpRequestBase {
 
   getResponseHeader(header: string): ?string {
     if (this.responseHeaders) {
-      var value = this.responseHeaders[header];
+      var value = this.responseHeaders[header.toLowerCase()];
       return value !== undefined ? value : null;
     }
     return null;
@@ -132,7 +132,12 @@ class XMLHttpRequestBase {
       return;
     }
     this.status = status;
-    this.responseHeaders = responseHeaders || {};
+    // Headers should be case-insensitive
+    var lcResponseHeaders = {};
+    for (var header in responseHeaders) {
+      lcResponseHeaders[header.toLowerCase()] = responseHeaders[header];
+    }
+    this.responseHeaders = lcResponseHeaders;
     this.responseText = responseText;
     this._setReadyState(this.DONE);
     this._sendLoad();