for and tablerow tag arguments

[jg-rp]

Python Liquid's {% for %} and {% tablerow %} tags only accept integers (and variables that resolve to integers) as values for cols, offset and limit. String representations of numbers and floats should be acceptable too.

If a string arguments is given and that string can not be cast to an integer, a default value is used. The default value differs for each argument and/or tag.

There's also some inconsistent behaviour in the reference implementation when handling objects that do not have a to_i method. This requires further investigation.