Fix unrecognized selector crash when adding metadata

[fractalwrench]

Goal

Prevents an unrecognized selector crash when adding metadata to a report in a beforeSend block.

The root cause of this bug is that an NSDictionary can be a tab value in the metadata property, whereas we assume that this is always an NSMutableDictionary. Adding a new attribute therefore triggers a crash, because bugsnag-cocoa attempts to mutate an immutable dictionary here: https://github.com/bugsnag/bugsnag-cocoa/blob/master/Source/BugsnagCrashReport.m#L396

Setting a user then starting a session provides one way to manifest this bug, as this uses an NSDictionary rather than NSMutableDictionary when deserializing crash reports. If an attribute is then added to an existing tab (e.g. app, device, user) in a beforeSend callback, this will crash the app: https://github.com/bugsnag/bugsnag-cocoa/blob/master/Source/BugsnagCrashReport.m#L259

private func setUpBugsnag() {
    let configuration = BugsnagConfiguration()
    configuration.add(beforeSend: self.willSendReport)
    configuration.setUser("my-user-123", withName: nil, andEmail: nil)
    Bugsnag.start(with: configuration)
    Bugsnag.startSession()
}

private func willSendReport(_ rawData: [AnyHashable: Any], report: BugsnagCrashReport) -> Bool {
    report.addMetadata(["foo": "bar"], toTabWithName: "user")
    report.addAttribute("session", withValue: "foobar", toTabWithName: "user")
    return true
}

Changeset

Converted the tab value to an NSMutableDictionary if it was not already of this type.

Tests

Manually verified that a report is now sent to the dashboard with the example case.

[rebello95]

Thanks for fixing! Can we add a unit test for this as well to validate it no longer crashes and prevent similar regressions?

[kattrali]

Merged #430 with some changes from this PR as well. Thanks!