Don't report zero units for calc expressions

Zeros in these expressions need to have units in order to work.

Fixes sds#843

lib/scss_lint/linter/zero_unit.rb

@@ -7,6 +7,7 @@ class Linter::ZeroUnit < Linter
 
     def visit_script_string(node)
       return unless node.type == :identifier
+      return if node.value.start_with?('calc(')
 
       node.value.scan(ZERO_UNIT_REGEX) do |match|
         next unless zero_with_length_units?(match.first)
@@ -21,6 +22,12 @@ def visit_script_number(node)
       add_lint(node, MESSAGE_FORMAT % length)
     end
 
+    def visit_script_funcall(node)
+      # Don't report errors for values within `calc` expressions, since they
+      # require units in order to work
+      yield unless node.name == 'calc'
+    end
+
   private
 
     ZERO_UNIT_REGEX = /

spec/scss_lint/linter/zero_unit_spec.rb

@@ -110,4 +110,14 @@
 
     it { should_not report_lint }
   end
+
+  context 'when calc expression with zero value has units' do
+    let(:scss) { <<-SCSS }
+      p {
+        width: calc(0px + 1.5em);
+      }
+    SCSS
+
+    it { should_not report_lint }
+  end
 end