Fix crash when adding BugsnagReactNativePlugin

[nickdowell]

Goal

Fix crash when starting Bugsnag with BugsnagReactNativePlugin.

BugsnagClient expects self.configuration.plugins to be of type NSMutableSet as advertised.

bugsnag-cocoa/Bugsnag/Client/BugsnagClient.m

Lines 253 to 256 in 96f738f

	id<BugsnagPlugin> reactNativePlugin = [NSClassFromString(@"BugsnagReactNativePlugin") new];
	if (reactNativePlugin) {
	[self.configuration.plugins addObject:reactNativePlugin];
	}

This bug has prevented updating bugsnag-cocoa in bugsnag-js

Changeset

Ensure plugins is a mutable set when copying a BugsnagConfiguration.

Testing

Amends unit test to replicate problem and verify fix.

[github-actions]

Bugsnag.framework binary size did not change - 820,608 bytes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%      +8  +0.1%      +8    __DATA,__objc_selrefs
  +0.0%      +8  +0.0%      +8    __TEXT,__text
  -0.1%      -8  -0.1%      -8    [__DATA]
  -0.1%      -8  -0.1%      -8    [__TEXT]
  [ = ]       0  [ = ]       0    TOTAL

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