[Fixes #1429] Content negotiation does a case-sensitive matching of media type's parameter values

@harshgMSFT 
Alternatively you could do `String.Equals(parameterValue2, parameters1[parameterKey], StringComparison.OrdinalIgnoreCase)` and drop the null check. Note that this would return true if both values are null. Depending on the context, that might not be a realistic scenario.