don't convert mailer default values to procs

Invoke mailer defaults as procs only if they are procs, do not convert
with to_proc.  That an object is convertible to a proc does not mean it's
meant to be always used as a proc.  Fixes #11533
text_area should handle nil value option like text_field
Simplify select_one method.

The select_all method always returns ActiveRecord::Result instance,
and the ActiveRecord::Result includes Enumerable mixin.
If #empty?, #first method returns nil. Thus we can simplify this method.
Fix indentation.
Merge pull request #11570 from joelcogen/harmonize_text_field_and_area

text_area should handle nil value option like text_field
Merge pull request #11579 from kennyj/refactor_20130724

Simplify select_one method.
Merge pull request #11559 from bughit/11533_to_proc

don't convert mailer default values to procs