Fix LogEntries: collection was modified exception

[StefH]

#308

[codecov]

Codecov Report

Merging #309 into master will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #309   +/-   ##
=======================================
  Coverage   79.94%   79.94%           
=======================================
  Files         110      110           
  Lines        4379     4379           
=======================================
  Hits         3501     3501           
  Misses        878      878

Impacted Files	Coverage Δ
src/WireMock.Net/Mapping.cs	100% <ø> (ø)	⬆️
src/WireMock.Net/Server/FluentMockServer.Admin.cs	61% <ø> (ø)	⬆️
...WireMock.Net/Server/FluentMockServer.LogEntries.cs	16.66% <100%> (ø)	⬆️
src/WireMock.Net/Owin/WireMockMiddleware.cs	94.44% <50%> (ø)	⬆️

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