fix: Support adding pre-delivery metadata to OOM reports

[kattrali]

It is currently not possible to append additional metadata in before send callbacks, only override completely, which is different than the normal behavior. This change amends the behavior to ensure that adding metadata works as expected.

Fixes #392

[codecov]

Codecov Report

Merging #393 into master will decrease coverage by 0.69%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #393     +/-   ##
=========================================
- Coverage   51.91%   51.21%   -0.7%     
=========================================
  Files          66       66             
  Lines        8937     8938      +1     
=========================================
- Hits         4640     4578     -62     
- Misses       4297     4360     +63

Impacted Files	Coverage Δ
Source/BugsnagCrashReport.m	82.34% <0%> (-0.17%)	⬇️
...SCrash/Recording/Tools/BSG_KSCrashCallCompletion.m	0% <0%> (-100%)	⬇️
Source/BugsnagErrorReportApiClient.m	13.33% <0%> (-73.34%)	⬇️
Source/BugsnagLogger.h	66.66% <0%> (-33.34%)	⬇️
...rce/KSCrash/Source/KSCrash/Recording/BSG_KSCrash.m	36.5% <0%> (-10.27%)	⬇️
...rash/Source/KSCrash/Recording/Tools/BSG_KSLogger.h	50% <0%> (-10%)	⬇️
Source/BugsnagSessionTrackingApiClient.m	78.72% <0%> (-8.52%)	⬇️
Source/BugsnagFileStore.m	60% <0%> (-7.15%)	⬇️

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