Map.Entry instances can have null values prior to Java 6

Test failure: http://build.clojure.org/job/test.generative/1/org.clojure$test.generative/console

Discussion: http://concurrency.markmail.org/message/pbruo2uxgur6wkoo?q=map%2Eentry+null&page=3#query:map.entry%20null+page:3+mid:z5arnbbzl2k32jda+state:results

Note that the fix must hold val in a local variable, because cache.remove will bomb if the value is null.