Fix be_routable description -- Renames BeRoutableMatcher to BeRoutable

[tonyta]

Problem
Previously, using be_routable in a one-liner...

RSpec.describe 'a not-routable route', type: :routing do
  let(:sad_route) { '/this/route/makes/me/sad' }
  specify { expect(get: sad_route).not_to be_routable }
end

...will output the following:

a not-routable route
  should not be routable matcher

BaseMatcher#description turns BeRoutableMatcher into "be routable matcher".

Solution
The solution implemented here is to rename BeRoutableMatcher to BeRoutable in order to rely on BaseMatcher#description to correctly return the following description:

a not-routable route
  should not be routable

This looks like the precedence in other matchers like BeValid.

Alternate Solution
Alternatively, #description could be overridden in BeRoutableMatcher:

def description
  "be routable"
end

I'd be happy to change PR if that's preferred 😸

[JonRowe]

I'm not sure what @cupakromer will think but personally I'd rather see:

Alternate Solution
Alternatively, #description could be overridden in BeRoutableMatcher:

def description
  "be routable"
end

As this matcher is named inline with the rest of them in this file, and potentially people are using the constant name (although it is marked private).

[cupakromer]

I agree with @JonRowe. Let's define description.

[tonyta]

Thanks @JonRowe @cupakromer. I'm closing this PR and redefining #description in #1310