[Bug fix]Fix culture for cobertura xml report

[MarcoRossignoli]

Fixes #463

@boldt comes from Germany where culture uses comma as decimal separator(like Italy).
This PR fix conversion using InvariantCulture that is defined as
The invariant culture is culture-insensitive; it is associated with the English language but not with any country/region.
doc https://docs.microsoft.com/en-us/dotnet/api/system.globalization.cultureinfo.invariantculture?view=netcore-2.2

Found this useful guide on formats...english languages uses dot. https://docs.oracle.com/cd/E19455-01/806-0169/overview-9/index.html

cc: @tonerdo

[MarcoRossignoli]

@tonerdo remember to change the merge strategy to "squash merge" to make easy possible reverts.