JSON processing should accept all data

[grantcodes]

At the moment the processJSONencodedBody function strips out anything from the micropub request that is not in the properties and doesn't start with mp-, and I'm not sure if this is correct.

My use case is I am starting to implement collections which should be sent as an array of children. I'm not 100% sure if children will be the only other property to support or if there will be more in the future but it is a 3 line fix to pass on all unrecognised data in the micropub request: see this commit