Add test for String Serializer for #589

#589

First test is the String Serializer. One thing that I'm wondering about the StringSerializer is whether it should be able to handle ```null``` values. I took the test strings from ```stratosphere-core.src.test.java.eu.stratosphere.types.StringSerializationTest.java``` as the test strings for the StringSerializer.

It was able to handle ```new String[] {"a", "", "bcd", "jbmbmner8 jhk hj \n \t üäßß@µ", "", "non-empty"}```
 but not ```new String[] {"a", null, "", null, "bcd", null, "jbmbmner8 jhk hj \n \t üäßß@µ", null, "", null, "non-empty"}```.

I am not sure if this is intended, so I have kept the array that does not include null for now.