Revert "Fix all columns being treated as NOT NULL under JRuby and Rails 4."

This reverts commit 7e2c054ef6260a25be5f34dd12aa45653dc9b85f.

Conflicts:
	lib/active_record/connection_adapters/postgis_adapter/rails3/main_adapter.rb
Another try at fixing all columns being NOT NULL under JRuby and Rails 4

This replaces the approach taken in 7e2c054ef6260a25be5f34dd12aa45653dc9b85f
While it seemed like that might be a cleaner approach, it broke Rails 4
array column identification, without a super clean way to fix it. So
instead, we'll replicate how the JDBC adapter handles it.