Fix JSON NaN

Hi,

The attached patch fixes the JSON generation when dealing with NaN (not
a number), this makes the JSON parsing in the web browser succeed
(before it would get a "nan" which is not a valid JS value)

Chris

libs/web/luasrc/http.lua

@@ -333,7 +333,12 @@ function write_json(x)
 			write(" }")
 		end
 	elseif type(x) == "number" or type(x) == "boolean" then
+		if (x ~= x) then
+			-- NaN is the only value that doesn't equal to itself.   
+			write("Number.NaN")
+		else
 			write(tostring(x))
+		end
 	elseif type(x) == "string" then
 		write("%q" % tostring(x))
 	end