Fixed RDF::Literal::Double instantiation from "INF"^^xsd:double, "-IN…

…F"^^xsd:double, and "NaN"^^xsd:double.

The following all now work as expected:

    RDF::Literal("INF", :datatype => XSD.double).to_f   #=> Infinity
    RDF::Literal("-INF", :datatype => XSD.double).to_f  #=> -Infinity
    RDF::Literal("NaN", :datatype => XSD.double).to_f   #=> NaN

lib/rdf/model/literal/double.rb

@@ -24,10 +24,15 @@ def initialize(value, options = {})
       @string   = options[:lexical] if options.has_key?(:lexical)
       @string   = value if !defined?(@string) && value.is_a?(String)
       @object   = case
-        when value.is_a?(::String)    then Float(value) rescue nil
+        when value.is_a?(::String) then case value
+          when 'INF'  then 1/0.0
+          when '-INF' then -1/0.0
+          when 'NaN'  then 0/0.0
+          else Float(value) rescue nil
+        end
         when value.is_a?(::Float)     then value
         when value.respond_to?(:to_f) then value.to_f
-        else Float(value.to_s) rescue nil
+        else Float(value.to_s) rescue nil # FIXME
       end
     end