Fix BodyAsFile to also allow relative paths

[StefH]

#243

[codecov]

Codecov Report

Merging #244 into master will increase coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
+ Coverage   89.64%   89.88%   +0.23%     
==========================================
  Files         102      102              
  Lines        5014     5099      +85     
==========================================
+ Hits         4495     4583      +88     
+ Misses        519      516       -3

Impacted Files	Coverage Δ
src/WireMock.Net/Server/FluentMockServer.Admin.cs	86.43% <100%> (+0.12%)	⬆️
...rc/WireMock.Net/Owin/Mappers/OwinResponseMapper.cs	100% <100%> (ø)	⬆️
...rc/WireMock.Net/Handlers/LocalFileSystemHandler.cs	100% <100%> (ø)	⬆️
src/WireMock.Net/Server/FluentMockServer.cs	91.46% <100%> (+0.02%)	⬆️
src/WireMock.Net/Owin/AspNetCoreSelfHost.cs	100% <100%> (ø)	⬆️
src/WireMock.Net/Owin/WireMockMiddlewareOptions.cs	100% <100%> (ø)	⬆️
src/WireMock.Net/ResponseBuilders/Response.cs	96.46% <100%> (+0.72%)	⬆️
src/WireMock.Net/Logging/WireMockNullLogger.cs	100% <0%> (+20%)	⬆️

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