Fixup for CASSANDRA-6914: null handling, ...

patch by slebresne; reviewed by thobbs for CASSANDRA-7155
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/cql3/ColumnCondition.java
	src/java/org/apache/cassandra/cql3/statements/CQL3CasConditions.java
	src/java/org/apache/cassandra/cql3/statements/ModificationStatement.java
	src/java/org/apache/cassandra/cql3/statements/SelectStatement.java
	src/java/org/apache/cassandra/cql3/statements/Selection.java
	src/java/org/apache/cassandra/db/marshal/CollectionType.java
	src/java/org/apache/cassandra/db/marshal/DynamicCompositeType.java
	src/java/org/apache/cassandra/db/marshal/ListType.java
	src/java/org/apache/cassandra/db/marshal/MapType.java
	src/java/org/apache/cassandra/db/marshal/SetType.java
	src/java/org/apache/cassandra/serializers/CollectionSerializer.java
	src/java/org/apache/cassandra/serializers/ListSerializer.java
	src/java/org/apache/cassandra/serializers/MapSerializer.java
	src/java/org/apache/cassandra/serializers/SetSerializer.java
	test/unit/org/apache/cassandra/db/marshal/CollectionTypeTest.java