Improve path parsing in _default_render_options

[jhawthorn]

This improves the path parsing here to be more strict, understand variants, and should support the additional validations around format in Rails 6.0.

I added a few previously failing specs, which this makes pass.

Fixes #2114

cc @samphippen @benoittgt @jaredmoody

[fables-tales]

@jhawthorn can you take a look at the build failures here? https://travis-ci.org/rspec/rspec-rails/jobs/526608975

[benoittgt]

It seems #symbols was added in Rails 5 and changed slightly after.

We have also a weird syntax issue. https://travis-ci.org/rspec/rspec-rails/jobs/526609025

SyntaxError:
  /home/travis/build/rspec/rspec-rails/spec/rspec/rails/example/view_example_group_spec.rb:150: syntax error, unexpected ':', expecting tASSOC
  ... :handlers=>['erb'], variants: ['fancy']}, {}, nil])
                                ^
  /home/travis/build/rspec/rspec-rails/spec/rspec/rails/example/view_example_group_spec.rb:150: syntax error, unexpected '}', expecting kEND
  ...>['erb'], variants: ['fancy']}, {}, nil])
                                ^
  /home/travis/build/rspec/rspec-rails/spec/rspec/rails/example/view_example_group_spec.rb:158: syntax error, unexpected kELSE, expecting kEND
  /home/travis/build/rspec/rspec-rails/spec/rspec/rails/example/view_example_group_spec.rb:183: syntax error, unexpected kDO_BLOCK, expecting kEND
  /home/travis/build/rspec/rspec-rails/spec/rspec/rails/example/view_example_group_spec.rb:184: syntax error, unexpected kDO_BLOCK, expecting kEND
  /home/travis/build/rspec/rspec-rails/spec/rspec/rails/example/view_example_group_spec.rb:199: syntax error, unexpected kDO_BLOCK, expecting kEND
  /home/travis/build/rspec/rspec-rails/spec/rspec/rails/example/view_example_group_spec.rb:200: syntax error, unexpected kDO_BLOCK, expecting kEND
  /home/travis/build/rspec/rspec-rails/spec/rspec/rails/example/view_example_group_spec.rb:221: syntax error, unexpected kEND, expecting $end