Add Subject.hasHashCode(int) and Subject.hasToString(String)

[PhilippWendler]

Sometimes the hashCode() of a class implementing an interface is important and should be tested (for example for collections, Map.Entry). In such cases it would be nice to have Subject.hasHashCode() instead of having to write assertThat(obj.hashCode()).isEqualTo(h) because the former could provide a better failure message including the value of obj.toString().

[kluever]

If we add this, we'd also probably want to add:
Subject.hasToString(String)

[kluever]

We discussed this pretty extensively, and we're deciding to not take any action here.

All of the assertions about hashcodes we could find inside of google, were of the form:
assertEquals(a.hashCode(), b.hashCode());

These use cases are much better supported by Guava's EqualsTester:
https://code.google.com/p/guava-libraries/source/browse/guava-testlib/src/com/google/common/testing/EqualsTester.java