modules/http: fix finding the static files

I also verified there's no other usage of the `moduledir` symbol from
lua.  Bug introduced in 2f81b111 (within !298).

NEWS

+Knot Resolver 1.3.1 (2017-06-xx)
+================================
+
+Bugfixes
+--------
+- modules/http: fix finding the static files (bug from 1.3.0)
+
+
 Knot Resolver 1.3.0 (2017-06-13)
 ================================
 

modules/http/http.lua

@@ -34,9 +34,10 @@ local mime_types = {
 -- Preload static contents, nothing on runtime will touch the disk
 local function pgload(relpath, modname)
 	if not modname then modname = 'http' end
-	local fp, err = io.open(string.format('%s/%s/%s', moduledir, modname, relpath), 'r')
+	local mdir = moduledir()
+	local fp, err = io.open(string.format('%s/%s/%s', mdir, modname, relpath), 'r')
 	if not fp then
-		fp, err = io.open(string.format('%s/%s/static/%s', moduledir, modname, relpath), 'r')
+		fp, err = io.open(string.format('%s/%s/static/%s', mdir, modname, relpath), 'r')
 	end
 	if not fp then error(err) end
 	local data = fp:read('*all')