Simple shape functions not counted in coverage when precompiled

When calling name on a shape, like this:
```julia
name(shape::Circle) = "Circle"
```
coverage says that it is [not covered](https://coveralls.io/builds/17369411/source?filename=src/shapes/circle.jl#L11) but it is [called in tests](https://github.com/jondea/MultipleScattering.jl/blob/7c0164906919b64569041882d78a21845f26f937/test/shapetests.jl#L25). I also have the same problem when using coverage locally. However, we did not have this problem before we enabled `precompile`.

I think this is a Julia bug, and so should be reported there, but I'm a bit nervous that I'm wrong...