Fix annotation for columns with Date and DateTime default values (#61)

In #58, it changed the default
behavior for unhandled types from `#inspect` to `#to_s`. I didn't
consider to add test cases for dates or datetimes which led to a
regression.

I tried writing unit tests for it, but realized that it wouldn't work
easily because Rails monkey patches `#inspect`:


https://github.com/rails/rails/blob/44bd6e7acaa91dd59c260beea6863a49b84f6039/activesupport/lib/active_support/core_ext/date/conversions.rb#L63-L67

Testing this behavior will be handled in a separate PR.

Author: drwl

lib/annotate_rb/model_annotator/column_annotation/default_value_builder.rb

@@ -36,7 +36,7 @@ def quote(value)
           when BigDecimal then value.to_s("F")
           when String then value.inspect
           else
-            value.to_s
+            value.inspect
           end
         end