Writer should use code/iso8601 for date/time/datetime, not inspect

[garysweaver]

To resolve #43 and #62, this converts Date, Time, DateTime (and ActiveSupport::TimeWithZone if exists) to code that creates a date using the iso8601 version of each time.

This could use some optimization. A much faster way is just to override inspect on Date, Time, DateTime (and ActiveSupport::TimeWithZone if exists) to return "(the class name).iso8601(#{iso8601.inspect})". However, this patch avoids inspect overrides, and it is doubtful most would want inspect overriden. It is ~3x slower than before. (4043 records took 8.1 sec vs. 2.4 sec and 79808 records took 83 sec vs. 30 sec).

Another option might be to use inspect but only temporarily override inspect on the affected classes (Date, Time, etc.), but I'm not absolutely sure that would work.