Prevent IE to cache requests/responses

Summary:
https://bitgoinc.atlassian.net/browse/BG-914
There is an issue with IE where you login/logout and try to login again and in the OTP screen throws an error, this is due to IE caching the requests/responses.
We need this special If-Modified-Since attribute in the header with a very old date so IE don't to that

This was causing the sdk to throw invalid hmac, because it was using an older HMAC for a new request, and thus throwing the error in the client.

Reviewers: arik

Reviewed By: arik

Subscribers: ben

Differential Revision: https://phabricator.bitgo.com/D6035

Author: RobertBaron

package.json

@@ -1,6 +1,6 @@
 {
   "name": "bitgo",
-  "version": "3.4.8",
+  "version": "3.4.9",
   "description": "BitGo Javascript SDK",
   "main": "./src/index.js",
   "keywords": [

src/bitgo.js

@@ -258,6 +258,8 @@ var BitGo = function(params) {
         }
 
         this.set('BitGo-Auth-Version', '2.0');
+        // prevent IE from caching requests
+        this.set('If-Modified-Since', 'Mon, 26 Jul 1997 05:00:00 GMT');
         if (bitgo._token) {
 
           // do a localized data serialization process