Add switch_to_test for erb templates to switch to rspec view specs.

Hi,

since I'm using erb a lot together with rspec's view specs, I added the feature to be able to switch between erb and spec.

Additionally I fixed the haml switcher. I'm currently not using haml a lot, but looking at the implementation in line 249, which switches from .haml TO .haml_spec.rb, the other way should switch from .haml_spec.rb, not from _haml.spec.rb.

You may decide.