MoneyMismatchException:contextMismatch when using different context

[BertKooij]

When using this library with a Money instance with a custom context calculating the inclusive/exclusive price will throw an exception.

The monies do not share the same context. If this is intended, use plus($money->toRational()) instead of plus($money).

In my use case the context should not round the numbers which could be achieved with a custom context. The default context auto rounds the values to the default rounding of the used currency.

[toonvandenbos]

Thanks for your PR. Released under version 2.0.1.