If an Unknown message is being constructed for sending, the content
may be empty, since the msgtype and body are exposed as C++ properties.
Previously, the to_json function assumed that the content could be
parsed as valid json, when in fact an empty content would throw an error.

Clang Format SPAAAAACES

Rename RelatesTo for reply messages

That's what the README suggests and - more importantly - the cmake
config file works with.
Also use angle brackets instead of quotation marks in two cases.