directly pass strings to service when serialization datetime objects

transferred from https://github.com/Azure/autorest.python/issues/1413

Right now in our vendored msrest code, we silently fail and drop incorrect datetime strings when serializing request bodies (see issue [here](https://github.com/Azure/azure-sdk-for-python/issues/25946)).

What we want to do instead is when serializing a datetime / date / duration etc, is if users pass in a string, we directly pass the string to the service. If they pass a string, that means they serialized it themselves, and we should trust them and rely on the server to do validation.