There are many 'rules' on the net saying that a comma should be placed before the relative pronoun 'which' in a non-restrictive clause. (http://www.oxforddictionaries.com/words/relative-clauses)
But would the sentence have the same meaning without the comma before the relative pronoun?
Does omitting a comma before a relative pronoun change a nonrestrictive clause into a restrictive clause?
'We ate the pizza, which was nice.'
'We ate the pizza which was nice.'
If so does the same apply to relative pronouns other than 'which'?