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
Merge pull request #176 from kgriffs/issues/155

fix(Request.client_accepts): Accept parsing doesn't handle application/*
feat(util.to_query_str): Smart handling of lists

This patch improves to_query_str so it can recognize instances of
list and convert it to a comma-delimited string value.

Closes #152