Publish over gRPC does not respect content type for non Json payload

[artursouza]

Expected Behavior

Users can publish XML and have the datacontenttype set to "text/xml" in cloudevent.

Actual Behavior

datacontenttype defaults to something else from Dapr runtime and is ignored.

Steps to Reproduce the Problem

Publish xml content and set "content-type" metadata to "text/xml". See that subscribers don't get the right value in datacontentype attribute in CloudEvent.

Release Note

RELEASE NOTE: FIXED datacontenttype to respect content-type metadata.