Do not use captital case for properties in message schema

[ts5746]

Original internal issue suggested to follow the guidelines for REST APIs in the JSON message schema, for example: https://hackernoon.com/restful-api-designing-guidelines-the-best-practices-60e1d954e7c9

[ts5746]

In general we to comply such guidelines and conventions. However, in this case the names in the JSON message schema that are exactly as named and defined in the standard, start with a capital on purpose, namely to distinguish them from other, non-standard object names. Also, the current situation is in line with the IETF JSON specification in RFC 7159 that actually includes examples with properties using capital case.. See also this discussion: https://stackoverflow.com/questions/5543490/json-naming-convention.