Improve HasEnv specs:

* Use a more classic style, and rely on a simple Hash instead of
  stubbing a double;
* Improve examples to show the intent more clearly (like the use of a
  symbol when querying).

spec/producer/core/tests/has_env_spec.rb

@@ -4,38 +4,39 @@ module Producer::Core
   module Tests
     describe HasEnv do
       let(:env)           { Env.new }
-      let(:variable_name) { :some_variable_name }
+      let(:variable_name) { 'SOME_VAR' }
+      let(:remote_env)    { { 'SOME_VAR' => 'SOME_VALUE' } }
       subject(:has_env)   { HasEnv.new(env, variable_name) }
 
       it_behaves_like 'test'
 
-      describe '#verify' do
-        let(:environment) { double 'environment' }
-
       before do
-          allow(env.remote).to receive(:environment) { environment }
+        allow(env.remote).to receive(:environment) { remote_env }
       end
 
-        it 'stringifies the queried variable name' do
+      describe '#verify' do
-          expect(environment).to receive(:key?).with(kind_of(String))
+        context 'remote environment var is defined' do
-          has_env.verify
+          it 'returns true' do
-        end
-
-        it 'upcases the queried variable name' do
-          expect(environment).to receive(:key?).with('SOME_VARIABLE_NAME')
-          has_env.verify
-        end
-
-        it 'returns true when remote environment var is defined' do
-          allow(environment).to receive(:key?) { true }
             expect(has_env.verify).to be true
           end
 
-        it 'returns false when remote environment var is not defined' do
+          context 'when var name is given as a lowercase symbol' do
-          allow(environment).to receive(:key?) { false }
+            let(:variable_name) { :some_var }
+
+            it 'returns true' do
+              expect(has_env.verify).to be true
+            end
+          end
+        end
+
+        context 'remote environment var is not defined' do
+          let(:variable_name) { 'SOME_NON_EXISTENT_VAR' }
+
+          it 'returns false' do
             expect(has_env.verify).to be false
           end
         end
       end
     end
+  end
 end