fix: Make metadata tabs always mutable

[snmaynard]

This PR addresses an issue whereby a tab within metadata can be an NSDictionary rather than an NSMutableDictionary, and an exception can be generated in a beforeSend callback, preventing the report being sent. The dictionaries in metadata can be immutable because within the constructor of a report we don't use the property setter or call through to BSGSanitizeDict which will ensure the dictionary is mutable. See https://github.com/bugsnag/bugsnag-cocoa/blob/master/Source/BugsnagCrashReport.m#L280 for an example.

The fix I have gone for here is to make a mutableCopy of the tab within the addAttribute:withValue:toTabWithName function. We could also add some calls to BSGSanitizeDict inside the constructor when initially setting the metadata value, which will ensure the dictionary is mutable, but I don't know the context of why that is not already there. Maybe @kattrali had a reason to avoid the setter when constructing the class. We could add the santize call also if we think that is a good idea for a belt and braces approach.

I have also added a try-catch to the execution of each beforeSend callback to ensure that if they throw an exception, the result is just that the beforeSend don't execute, rather than preventing delivery of the reports.

It is worth pointing out that I believeaddMetadata:toTabWithName: was not affected by this issue and so is unchanged.

The tests have also been paired back to be the bare minimum to reproduce the issue and verify the code change fixes it. This PR is an alternative to #429.