Make datacontenttype value check case-insensitive

SDK is using case-sensitive checks when formatting CloudEvents to Json based on "datacontenttype" value. MIME types type and subtype are case insensitive according to the spec.

For example, this code snippet throws exception:
System.ArgumentException: 'JsonEventFormatter cannot serialize data of type System.String with content type 'application/JSON''

CloudEvent cloudEvent = new CloudEvent
{
Id = "event-id",
Type = "event-type",
Source = new Uri("https://cloudevents.io/"),
Time = DateTimeOffset.UtcNow,
DataContentType = "application/JSON",
Data = "text"
};

CloudEventFormatter formatter = new JsonEventFormatter();
var httpContent = cloudEvent.ToHttpContent(ContentMode.Structured, formatter);

Start with JsonEventFormatter and the CloudEvent.DataContentType handling described in the report, then reproduce the application/JSON example. Trace the formatting path and verify that MIME type and subtype comparisons are case-insensitive; done means the example serializes successfully without changing unrelated content types.