Why is my LinkedHashMap merging key with value? - jenkins

Question

Intro
I wrote some code for a Jenkins pipeline in Groovy that looks like this:
def sendNotifications (args) {
// ... use args ...
sendSlack(
failOnError: true,
color: '#00FF00',
message: "foo",
channel: "#mychan"
)
}
// slackMessage not shown returns a string based on the args.
This code seemed to be working just fine in my pipeline. Now I want to test it. So I create a function that mocks out sendSlack because I don't want my test to send a Slack message, I want to check to see if the arguments passed to sendSlack are the expected ones. So I did this:
void testStubbedSendNotifications(Map inputArgs, ArrayList slackArgs) {
def n = new Notifier()
def sendSlackArgs = []
n.metaClass.sendSlack = { Map x -> sendSlackArgs << x }
n.sendNotifications(inputArgs)
assert sendSlackArgs == slackArgs
}
What is happening here is simple. Instead of calling sendSlack, I'm pushing the arguments that would be called onto an ArrayList and then I check to make sure that list looks right in an assert. So this function takes arguments to call sendNotifications with as well as a list of expected values to see sendSlack being called with as a result.
The Problem
My expected values get mangled only in my tests. Instead of the arguments being a hash map like this:
[
failOnError: true,
color: '#00FF00',
message: "foo",
channel: "#mychan"
]
I instead get this:
[
failOnError: true,
color: '#00FF00',
"message:foo" : null,
channel: "#mychan"
]
Why does the message's key and value get merged into the key?