Fix milliseconds format in configuration changes operation date

[5:56 PM] Harald Pehl: @here I noticed that the operation-date attribute for /subsystem=core-management/service=configuration-changes:list-changes has sometimes a millisecond part, sometimes not
[5:56 PM] Harald Pehl:
[standalone@localhost:9991 /] /subsystem=core-management/service=configuration-changes:list-changes
{
"outcome" => "success",
"result" => [
{
"operation-date" => "2018-02-14T16:42:09.856Z",
"access-mechanism" => "HTTP",
"remote-address" => "/172.18.0.1",
"outcome" => "success",
"operations" => [{
"blocking" => false,
"operation" => "reload",
"address" => [],
"operation-headers" =>

{ "access-mechanism" => "HTTP", "caller-type" => "user" }
}]
},
{
"operation-date" => "2018-02-14T16:34:20Z",
"access-mechanism" => "HTTP",
"remote-address" => "/172.18.0.1",
"outcome" => "success",
"operations" => [{
"operation" => "composite",
"address" => [],
"steps" => [{ "name" => "statistics-enabled", "value" => true, "operation" => "write-attribute", "address" => [("subsystem" => "undertow")] }],
"operation-headers" => { "access-mechanism" => "HTTP", "caller-type" => "user" }

}]
}
]
}
[5:56 PM] Harald Pehl: This makes it very hard to parse the date in the client