Surround class values with spaces.

When using a CSS class selector the resulting XPath string passed to contains()
should be surrounded by spaces.

lib/oga/css/parser.y

@@ -103,7 +103,7 @@ rule
           :call,
           'contains',
           s(:call, 'concat', s(:string, ' '), axis, s(:string, ' ')),
-          s(:string, val[1])
+          s(:string, " #{val[1]} ")
         )
       }
     ;

spec/oga/css/parser/classes_spec.rb

@@ -4,27 +4,27 @@ describe Oga::CSS::Parser do
   context 'classes' do
     example 'parse a class selector' do
       parse_css('.foo').should == parse_xpath(
-        'descendant-or-self::*[contains(concat(" ", @class, " "), "foo")]'
+        'descendant-or-self::*[contains(concat(" ", @class, " "), " foo ")]'
       )
     end
 
     example 'parse a selector for an element with a class' do
       parse_css('foo.bar').should == parse_xpath(
-        'descendant-or-self::foo[contains(concat(" ", @class, " "), "bar")]'
+        'descendant-or-self::foo[contains(concat(" ", @class, " "), " bar ")]'
       )
     end
 
     example 'parse a selector using multiple classes' do
       parse_css('.foo.bar').should == parse_xpath(
-        'descendant-or-self::*[contains(concat(" ", @class, " "), "foo") ' \
-          'and contains(concat(" ", @class, " "), "bar")]'
+        'descendant-or-self::*[contains(concat(" ", @class, " "), " foo ") ' \
+          'and contains(concat(" ", @class, " "), " bar ")]'
       )
     end
 
     example 'parse a selector using a class and an ID' do
       parse_css('#foo.bar').should == parse_xpath(
         'descendant-or-self::*[@id="foo" and ' \
-          'contains(concat(" ", @class, " "), "bar")]'
+          'contains(concat(" ", @class, " "), " bar ")]'
       )
     end
   end

spec/oga/css/parser/ids_spec.rb

@@ -22,7 +22,7 @@ describe Oga::CSS::Parser do
 
     example 'parse a selector using an ID and a class' do
       parse_css('.foo#bar').should == parse_xpath(
-        'descendant-or-self::*[contains(concat(" ", @class, " "), "foo") ' \
+        'descendant-or-self::*[contains(concat(" ", @class, " "), " foo ") ' \
           'and @id="bar"]'
       )
     end