Correct definition of String#split

To be precise #split does not create a 'set' of strings, but an array of strings.
See official docs http://www.ruby-doc.org/core-2.1.0/String.html#method-i-split

>> puts "I am one long string".split(" ").class
=> Array
Merge pull request #839 from monthypython/patch-1

Correct definition of String#split