Rename default_form_builder to avoid collision

[jpcody]

The Configuring Action View guides say the following:

config.action_view.default_form_builder tells Rails which form builder to use by default. The default is ActionView::Helpers::FormBuilder. If you want your form builder class to be loaded after initialization (so it's reloaded on each request in development), you can pass it as a String

Unfortunately, when you set config.action_view.default_form_builder as a string, it is set as a class-level attr_accessor on ActionView::Base. Then line 1191 on master attempts to call an internal method to constantize this string, but instead, it receives the default_form_builder cattr_accessor as a string and attempts to call new on it on the subsequent line.

This problem only appears when setting this configuration before the file is required, and quite honestly, I couldn't discern how to write a test for the case. But as it's only a naming collision, it seemed innocuous enough to submit as a PR.

Thoughts?