xpath abbreviate: add support for string literal that contains double-quote (#96)

This adds support for a string literal that contains a double-quote to
`XPathParser#abbreviate`. Basically any literal that contains a
double-quote `"` must be quoted by single-quotes `'` since XPath 1.0
does not support any escape characters.

The change improves the following test script
```ruby
require 'rexml'

parsed = REXML::Parsers::XPathParser.new.parse('/a[b/text()=concat("c\'",\'"d\')]')
puts "#{parsed}"
puts ""
appreviated = REXML::Parsers::XPathParser.new.abbreviate parsed
puts "#{appreviated}"
```
### Output Before Change
```
[:document, :child, :qname, "", "a", :predicate, [:eq, [:child, :qname, "", "b", :child, :text], [:function, "concat", [[:literal, "c'"], [:literal, "\"d"]]]]]

/a[ b/text() = concat( "c'" , "\"d" ) ]
```
### Output After Change
```
[:document, :child, :qname, "", "a", :predicate, [:eq, [:child, :qname, "", "b", :child, :text], [:function, "concat", [[:literal, "c'"], [:literal, "\"d"]]]]]

/a[ b/text() = concat( "c'" , '"d' ) ]
```

---------

Co-authored-by: Matt Pulver <[email protected]>

Author: pulver

lib/rexml/parsers/xpathparser.rb

@@ -178,7 +178,7 @@ def predicate_to_string( path, &block )
         when :literal
           path.shift
           string << " "
-          string << path.shift.inspect
+          string << quote_literal(path.shift)
           string << " "
         else
           string << " "
@@ -189,6 +189,21 @@ def predicate_to_string( path, &block )
       end
 
       private
+      def quote_literal( literal )
+        case literal
+        when String
+          # XPath 1.0 does not support escape characters.
+          # Assumes literal does not contain both single and double quotes.
+          if literal.include?("'")
+            "\"#{literal}\""
+          else
+            "'#{literal}'"
+          end
+        else
+          literal.inspect
+        end
+      end
+
       #LocationPath
       #  | RelativeLocationPath
       #  | '/' RelativeLocationPath?