Fix: reject midInProcess callback on close

[tayou89]

Summary

Fixes #53 — midInProcess callback is not invoked when close() is called, causing promises to hang forever.

• Reject midInProcess with the close error (or default "service unavailable") before nulling, using the same process.nextTick pattern as midQueue items
• Add tests for both error-forwarding and default-error scenarios
• Handle unhandled promise rejections in existing sendMid tests

The Problem

When close() is called, midQueue items get their callbacks properly rejected (lines 425-430), but midInProcess is silently set to null (line 432). Any pending callback/Promise for the in-flight MID hangs forever.

The Fix

if (this.midInProcess) {
    let e = err || new Error("service unavailable");
    let mid = this.midInProcess;
    this.midInProcess = null;
    process.nextTick(() => mid.doCallback(e));
} else {
    this.midInProcess = null;
}

Test plan

• New test: close(err) forwards the error to midInProcess callback
• New test: close() without args uses default "service unavailable"
• All existing tests pass (except pre-existing timeout in keepAlive test)
• Updated sendMid tests to handle the now-expected promise rejection