fix(Request.get_param_as_list): Handle missing list items

This patch modifies get_param_as_list to handle missing list items by
inserting None as a placeholder. If the query parameter's value is just
commas, then they are collapsed into a single None (as if the query
param hadn't been specified).

Fixes #154
Merge pull request #175 from kgriffs/issues/154

fix(Request.get_param_as_list): Handle missing list items