ApplicationName property isn't validated

[mmdriley]

The ApplicationName property has to be valid when included in a User-Agent value. We don't validate it eagerly, so users can see a FormatError later when we actually try to construct the request.

We can do the validation with ProductInfoHeaderValue.TryParse.

[chrsmith]

Perhaps this would be a good bug to work with @chrisdunelm to get fixed? So he can learn the dark mysteries of the client libraries?

[chrisdunelm]

Implementation rolled back in #1034 - validation was stricter than required, causing unnecessary validation failures.