Properly handle event payload members of blob and string type

[sugmanue]

Issue #, if available:

Description of changes:

Properly handle event payload members of blob and string type, those are serialized directly into the event payload using a different content-type header.

Serializes a member with the trait @eventPayload directly into the payload of the event. The following member types have special handling:

• blob For blob members the non-encoded bytes are the payload of the event and the event :content-type header is set to "application/octet-stream"
• string For string members the UTF-8 encoded bytes are the payload of the event and the event :content-type header is set to "text/plain"
• If the payload is empty, no :content-type is added as header of the message
• Any other is encoded with the underlying protocol and uses the content type defined for it, unless

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