Upgrade request dependency to fix NODE_DEBUG issue

If NODE_DEBUG contains 'request' (to enable debugging), the currently used version of request (2.27.0) has a [bug][request-659] that causes an exception. This bug is fixed in version 2.28.0 and later.

This bug is important to fix, because without it, end users that also use request in their own code cannot enable debugging for non-azure uses without breaking azure calls.

[request-659]: https://github.com/mikeal/request/pull/659<p><a href="https://github.com/Azure/azure-sdk-for-node/pull/1152">https://github.com/Azure/azure-sdk-for-node/pull/1152</a></p>
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