Update RateConverterTest.java

I don't understand very well the last test.
If on OANDA we have directly EURCHF why, in the test, pass through USD?

I mean on OANDA there's a direct conversion...

To sum up you test EUR to USD and USD to CHF but in reality I should directly do EUR to CHF, I think that this particular test could be wrong..you are testing something that in reality wouldn't happen