Fix tokenizing of defined? in Ruby lexer (#1247)

Due to the fact that `defined?` ends in a question mark, this was
causing issues with its tokenization. This commit fixes the error.

lib/rouge/lexers/ruby.rb

@@ -183,7 +183,7 @@ def self.detect?(text)
 
         mixin :strings
 
-        rule %r/(?:#{keywords.join('|')})\b/, Keyword, :expr_start
+        rule %r/(?:#{keywords.join('|')})(?=\W|$)/, Keyword, :expr_start
         rule %r/(?:#{keywords_pseudo.join('|')})\b/, Keyword::Pseudo, :expr_start
 
         rule %r(

spec/visual/samples/ruby

@@ -49,6 +49,35 @@ a/ b #comment
 x.a / 1 # comment
 Foo::a / 3 + 4
 
+########
+# Keywords that may be immediately followed by an opening
+# parenthesis
+########
+
+# defined?
+return if defined? Rouge
+return unless defined?(Foobar)
+
+# super
+class Beta < Alpha
+  def gamma(msg)
+    super
+    @msg = msg
+  end
+end
+
+class Bar < Foo
+  def log(msg)
+    super()
+  end
+end
+
+class Ipsum < Lorem
+  def dolor(txt)
+    super(txt)
+  end
+end
+
 ########
 # The popular . :bow:
 ########