Improve MapAssert.containsKey with varargs params

[joel-costigliola]

First rename to containsKeys

It would allow to write :

assertThat(elvesRingBearers).containsKeys(nenya, narya, vilya);

instead of

assertThat(elvesRingBearers).containsKey(nenya).containsKey(narya).containsKey(vilya);