allow multiple tax statements

[caifara]

We came across onix messages with multiple tax statements. This PR changes the price.tax call into price.taxes to support such cases.

[julbouln]

Hi,

Thank you for your contribution !

To avoid API break, it would be nice to add a singular tax method like for place https://github.com/immateriel/im_onix/blob/master/lib/onix/contributor.rb#L49

[caifara]

Hi, thanks for your quick response!

I wrote one but removed it later because it may create the impression there is just one tax statement. Multiple tax statements in our case are used when the price amount is split between multiple tax rates. So reading only one results in wrong information.

Would you be okay with a singular tax method that raises when multiple are available?