Express Route Port identity type returns with case changed

In API version 2020-05-01, the [`identity.type`](https://github.com/Azure/azure-rest-api-specs/blob/ace28fcf0a086afdacae6aee5c4fe92286197281/specification/network/resource-manager/Microsoft.Network/stable/2020-05-01/network.json#L210-L210) property of Express Route Port is defined to be `UserAssigned`, whilst on a `GET`, the `identity.type` in the response turns to be `userAssigned` (the first character changed to lower case).

Expected that the case keep consistent, as what is defined in Swagger.

Start with the identity.type definition in specification/network/resource-manager/Microsoft.Network/stable/2020-05-01/network.json at the linked line, then compare it with the Express Route Port GET response. Done means the response preserves the UserAssigned casing defined in the API specification.

Written by the indexing model from the issue text.