Prevent parsing comma in MySQL `SHOW CREATE TABLE` output

When a foreign key has no ON DELETE or additional options, and is followed by
a comma (i.e. the first of many), then the trailing comma was incorrectly
parsed as a constraint option.  This caused 'options: ","' to appear in
schema.rb.
Merge pull request #158 from domcleal/mysql-options-comma

Prevent parsing comma in MySQL `SHOW CREATE TABLE` output