Fix loading index summary containing empty key

patch by yukim; reviewed by jbellis for CASSANDRA-5965
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	src/java/org/apache/cassandra/io/sstable/IndexSummary.java
	test/unit/org/apache/cassandra/io/sstable/IndexSummaryTest.java