Fix UnnecessaryStringOutput on literal strings with method calls

`UnnecessaryStringOutput` would incorrectly report a lint on lines like
the following:

  = 'user'.pluralize(@users.count)

Fix it by actually parsing the script and linting if the resulting type
is a string or interpolated string.

Change-Id: Id35b9ce1e406df3252c54cd73e8c778785718b8a
Reviewed-on: https://gerrit.brigade.com/48481
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

CHANGELOG.md

@@ -12,6 +12,8 @@
 * Add `MultilineScript` linter to report scripts with trailing operators that
   should be merged with the following line
 * Add `AltText` linter to report missing `alt` attributes on `img` tags
+* Fix `UnnecessaryStringOutput` to not report warnings for strings with methods
+  called on them
 
 ## 0.11.0
 

lib/haml_lint/linter/unnecessary_string_output.rb

@@ -30,7 +30,10 @@ def visit_script(node)
     private
 
     def outputs_string_literal?(script_node)
-      %w[' "].include?(script_node.script.lstrip[0])
+      tree = parse_ruby(script_node.script)
+      [:str, :dstr].include?(tree.type)
+    rescue ::Parser::SyntaxError # rubocop:disable Lint/HandleExceptions
+      # Gracefully ignore syntax errors, as that's managed by a different linter
     end
   end
 end

spec/haml_lint/linter/unnecessary_string_output_spec.rb

@@ -53,4 +53,10 @@
 
     it { should_not report_lint }
   end
+
+  context 'when script outputs literal string with method called on it' do
+    let(:haml) { "= 'user'.pluralize(@users.count)" }
+
+    it { should_not report_lint }
+  end
 end