Add unit tests for DataFrame API: First, Head, Limit, Take and Distinct
Add unit tests for DataFrame API: First, Head, Limit, Take and Distinct

tawan0109/SparkCLR,hebinhuang/Mobius,dwnichols/Mobius,dwnichols/Mobius,danielli90/SparkCLR,Microsoft/SparkCLR,cyruszhang/SparkCLR,skaarthik/SparkCLR,skaarthik/Mobius,dwnichols/Mobius,skaarthik/Mobius,kai-zeng/Mobius,dwnichols/Mobius,kai-zeng/Mobius,kai-zeng/Mobius,skaarthik/Mobius,danielli90/SparkCLR,kai-zeng/Mobius,Microsoft/SparkCLR,skaarthik/SparkCLR,dwnichols/Mobius,danielli90/SparkCLR,skaarthik/Mobius,skaarthik/Mobius,hebinhuang/Mobius,danielli90/SparkCLR,hebinhuang/Mobius,hebinhuang/Mobius,cyruszhang/SparkCLR,xiongrenyi/SparkCLR,hebinhuang/Mobius,hebinhuang/Mobius,kai-zeng/Mobius,danielli90/SparkCLR,tawan0109/SparkCLR,skaarthik/SparkCLR,tawan0109/SparkCLR,tawan0109/SparkCLR,cyruszhang/SparkCLR,xiongrenyi/SparkCLR,skaarthik/Mobius,xiongrenyi/SparkCLR,tawan0109/SparkCLR,xiongrenyi/SparkCLR,xiongrenyi/SparkCLR,kai-zeng/Mobius,Microsoft/SparkCLR,dwnichols/Mobius