xpath: Fix normalize_space(array) case (#111)

GitHub: fix GH-110

Fixed a bug in `REXML::Functions.normalize_space(array)` and introduced
test cases for it:

- Corrected a typo in the variable name within the collect block
(`string` -> `x`).
- Added `test_normalize_space_strings` to `test/functions/test_base.rb`.

---------

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

lib/rexml/functions.rb

@@ -262,11 +262,10 @@ def Functions::string_length( string )
       string(string).length
     end
 
-    # UNTESTED
     def Functions::normalize_space( string=nil )
       string = string(@@context[:node]) if string.nil?
       if string.kind_of? Array
-        string.collect{|x| string.to_s.strip.gsub(/\s+/um, ' ') if string}
+        string.collect{|x| x.to_s.strip.gsub(/\s+/um, ' ') if x}
       else
         string.to_s.strip.gsub(/\s+/um, ' ')
       end

test/functions/test_base.rb

@@ -229,6 +229,28 @@ def test_normalize_space
       assert_equal( [REXML::Comment.new("COMMENT A")], m )
     end
 
+    def test_normalize_space_strings
+      source = <<-XML
+<a><b>breakfast     boosts\t\t
+ 
+concentration   </b><c>
+Coffee beans
+ aroma
+
+
+
+</c><d>        Dessert
+ \t\t    after    dinner</d></a>
+      XML
+      normalized_texts = REXML::XPath.each(REXML::Document.new(source), "normalize-space(//text())").to_a
+      assert_equal([
+                     "breakfast boosts concentration",
+                     "Coffee beans aroma",
+                     "Dessert after dinner",
+                   ],
+                   normalized_texts)
+    end
+
     def test_string_nil_without_context
       doc = REXML::Document.new(<<-XML)
       <?xml version="1.0" encoding="UTF-8"?>