Fix specs on windows

Again waiting to see if CI passes to check I haven't broken anything on linux...

I fixed several specs but four of them still fail, each with this error:
```ruby
middleman/middleman-core/lib/middleman-core/cli/build.rb:230:in `realpath': "\x81" to UTF-8 in conversion from Windows-1252 to UTF-8 (Encoding::UndefinedConversionError)
```
I don't know how to fix that, but at least the other ~350 specs are testable now.

RSpec may need a better dependency version too, will check the CI build...