Return the property value in an empty value rule set

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### Unreleased
 
+ * Improve exception message for missing value [#131](https://github.com/premailer/css_parser/pull/131)
  * `:rule_set_exceptions` option added [#132](https://github.com/premailer/css_parser/pull/132)
 
 ### Version 1.11.0

lib/css_parser/rule_set.rb

@@ -93,6 +93,8 @@ def []=(property, value)
         else
           declarations[property] = Value.new(value)
         end
+      rescue ArgumentError => e
+        raise e.exception, "#{property} #{e.message}"
       end
       alias add_declaration! []=
 

test/rule_set/declarations/test_value.rb

@@ -102,6 +102,12 @@ class RuleSetProperyTest < Minitest::Test
 
       assert_equal true, CssParser::RuleSet::Declarations::Value.new('value').value.frozen?
     end
+
+    it 'raises an exception when the value is empty' do
+      assert_raises ArgumentError do
+        CssParser::RuleSet::Declarations::Value.new
+      end
+    end
   end
 
   describe '#to_s' do

test/rule_set/test_declarations.rb

@@ -68,6 +68,14 @@ class RuleSetDeclarationsTest < Minitest::Test
 
       assert_equal declarations.method(:[]=), declarations.method(:add_declaration!)
     end
+
+    it 'raises an exception including the property when the value is empty' do
+      declarations = CssParser::RuleSet::Declarations.new
+
+      assert_raises ArgumentError, 'foo value is empty' do
+        declarations['foo'] = '!important'
+      end
+    end
   end
 
   describe '#[]' do