drop_duplicates() on a float64 type column containing NaN values will cause CPU 100% occupied forever
I'm using 0.15.0 version, and I find when I apply drop_duplicates() on "PostCode" column, it will occupy all the CPU forever, and I have to kill the process to stop it.

dataset = pd.read_csv(dataset_path, encoding = 'GB2312')
dataset['PostCode'].drop_duplicates()

"PostCode" column contains a list of integers but with some "NULL" string, I'm not sure if it's related to column dtype, which for "PostCode" is float64...but the interesting thing is there is another column "AreaCode" which is similar with "PostCode", containing a list of integers and some "NULL", and it works well with drop_duplicates()

Here is my test file download link: 
https://drive.google.com/file/d/0B501Knu4O8vZT1RyampvU2RNTms/view?usp=sharing
Which contains more than 300k rows data
