Improve small details in specs:

* Fix coding standards;
* Simplify some expectations (eq instead of be matcher);
* Expect identity on block instead of calling;
* Change some before call as oneliners;
* Avoid shadowing variable names;
* Improve wording where needed.

spec/producer/core/tests/has_env_spec.rb

@@ -11,7 +11,7 @@ module Producer::Core
     end
 
     describe '#verify' do
-      let(:environment) { double('environment') }
+      let(:environment) { double 'environment' }
 
       before do
         allow(env.remote).to receive(:environment) { environment }

spec/producer/core/tests/has_file_spec.rb

@@ -19,7 +19,7 @@ module Producer::Core
       end
 
       it 'returns the file existence' do
-        existence = double('existence')
+        existence = double 'existence'
         allow(env.remote.fs).to receive(:has_file?) { existence }
         expect(has_file.verify).to be existence
       end