Fix UnnecessaryStringOutput handling of nested content

The linter would incorrectly report lints for tags with nested content
containing interpolation, e.g.

  %tag
    Some #{interpolated} text

This is because the HAML parser treats the nested content as a script
node, even though it's not explicitly set as a script.

For now, include a special case in the linter. If more situations like
this come up, we'll add a special case to the `NodeTransformer` class.

Change-Id: I4000489c56d6e9b2f655a3c04154b43315bb64a2
Reviewed-on: http://gerrit.causes.com/42565
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

lib/haml_lint/linter/unnecessary_string_output.rb

@@ -18,6 +18,10 @@ def visit_tag(node)
     end
 
     def visit_script(node)
+      # Some script nodes created by the HAML parser aren't actually script
+      # nodes declared via the `=` marker. Check for it.
+      return if node.first_line_source !~ /\s*=/
+
       if outputs_string_literal?(node)
         add_lint(node, MESSAGE)
       end

spec/haml_lint/linter/unnecessary_string_output_spec.rb

@@ -44,4 +44,13 @@
     let(:haml) { "= 'hello world'" }
     it { should report_lint }
   end
+
+  context 'when tag contains nested text with interpolation' do
+    let(:haml) { <<-'HAML' }
+      %tag
+        Some #{interpolated} text
+    HAML
+
+    it { should_not report_lint }
+  end
 end