WIP: Fix spec type inferrence

Fix spec type inferrence.

- In #970 we added `infer_spec_type_from_file_location!` but forgot
  to remove the code in `lib/rspec/rails/configuration.rb` that
  made it always infer - so the opt-in API was there but it
  was always enabled. Here I've made it truly opt-in.
- The logic of `infer_spec_type_from_file_location!` also setup
  the helper module inclusion via explicit `:type` metadata but
  was only intended to setup the implicit mapping of spec file
  location to type.  Here I've made the module inclusion via
  `:type` always work w/o an explicit config option needed to
  enable it.
- We used to mutate the `:type` metadata on inclusion of the helper
  module, but this caused bugs such as #825. The problem is that
  rspec-core uses a raw hash for the metadata and doesn't re-apply
  filtering/inclusion logic when the metadata hash is mutated.
  Instead, we use a new explicit `define_derived_metadata` API.

Fixes #825 and closes #829.

/cc @JonRowe @cupakromer @alindeman @xaviershay