Support the 'a' keyword in SPARQL lexer (#1493)

The case-sensitive keyword `a` is a valid keyword in SPARQL. At
present, the SPARQL lexer does not recognise it as such. This commit
fixes that error (the rule is similar to the one used in the Turtle lexer).

lib/rouge/lexers/sparql.rb

@@ -35,20 +35,21 @@ def self.keywords
       state :root do
         rule %r(\s+)m, Text::Whitespace
         rule %r(#.*), Comment::Single
-        
+
         rule %r("""), Str::Double, :string_double_literal
         rule %r("), Str::Double, :string_double
         rule %r('''), Str::Single, :string_single_literal
         rule %r('), Str::Single, :string_single
-       
+
         rule %r([$?]\w+), Name::Variable
         rule %r((\w*:)(\w+)?) do |m|
           token Name::Namespace, m[1]
           token Str::Symbol, m[2]
         end
         rule %r(<[^>]*>), Name::Namespace
         rule %r(true|false)i, Keyword::Constant
-
+        rule %r/a\b/, Keyword
+
         rule %r([A-Z]\w+\b)i do |m|
           if self.class.builtins.include? m[0].upcase
             token Name::Builtin
@@ -101,7 +102,7 @@ def self.keywords
         end
         mixin :string_single_common
       end
-      
+
       state :string_single_literal do
         rule %r(''') do
           token Str::Single

spec/visual/samples/sparql

@@ -52,3 +52,11 @@ false
 '''The librarian said, "Perhaps you would enjoy 'War and Peace'."'''
 """The librarian said, 'Perhaps you would enjoy "War and Peace".'"""
 "\u4444 and \U88888888"
+
+# `a` keyword
+PREFIX : <http://example.org/#>
+SELECT ?item ?itemLabel
+WHERE
+{
+    ?item a :Test .
+}