Extract Task#template code in Template class

spec/fixtures/templates/basic.erb

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+basic template

spec/fixtures/templates/variables.erb

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+<%= @foo %>

spec/producer/core/template_spec.rb

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+require 'spec_helper'
+
+module Producer::Core
+  describe Template do
+    include FixturesHelpers
+
+    let(:path)          { 'basic' }
+    let(:search_path)   { fixture_path_for 'templates' }
+    subject(:template)  { described_class.new path, search_path: search_path }
+
+    describe '#render' do
+      it 'renders ERB templates' do
+        expect(template.render).to eq "basic template\n"
+      end
+
+      context 'when variables are given' do
+        let(:path) { 'variables' }
+
+        it 'declares given variables in ERB render binding' do
+          expect(template.render foo: 'bar').to eq "bar\n"
+        end
+      end
+
+      context 'when relative path is requested' do
+        let(:path) { fixture_path_for('templates/basic').insert 0, './' }
+
+        it 'does not enforce `template\' search path' do
+          expect(template.render).to eq "basic template\n"
+        end
+      end
+    end
+  end
+end