replace mutable parameters defaults with the None object. issue #492

[0xsirsaif]

Hi, this pull request resolves the problem of this issue #492. I changed the default parameters of the following methods:

• Row.__init__
• Dataset.insert
• Dataset.rpush
• Dataset.lpush
• Dataset.append
• Dataset.extend

[0xsirsaif]

Hi @claudep, could you review this for me?

[claudep]

Sorry for the delay, superseded by #494 (I kept you as author, of course).

[hugovk]

494 merged, thanks both!