revisit URL field encoding

[golnazads]

https://github.com/adsabs/export_service/blob/master/exportsrv/formatter/customFormat.py#L739

as per Dave Green dag@mrao.cam.ac.uk
'''
The & in the bibcode is not escaped with %/R, but I see that some
characters have been escaped in %/U ... but this has escaped the space
in "<A HREF=..." to "+", which is not correct I think as that space is
not in the URL itself, and the double quotation marks around the URL
have been change to "%23" (possibly again not correct?). But the "&" has
not been escaped to "%26".
'''