[FIX] base: binary url attachment on windows
[FIX] base: binary url attachment on windows

On windows when opening a binary file a 'b' flag is to be used:
 https://docs.python.org/2/tutorial/inputoutput.html#reading-and-writing-files

Before this commit, /web/image/* and /web/content/* routes serving
an ir.attachment with url could return empty or truncated content.

opw-693306

ygol/odoo,dfang/odoo,hip-odoo/odoo,storm-computers/odoo,bplancher/odoo,ygol/odoo,storm-computers/odoo,laslabs/odoo,dfang/odoo,laslabs/odoo,ygol/odoo,dfang/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,storm-computers/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,hip-odoo/odoo,bplancher/odoo,storm-computers/odoo,hip-odoo/odoo,dfang/odoo,laslabs/odoo,hip-odoo/odoo,dfang/odoo,laslabs/odoo,bplancher/odoo,storm-computers/odoo,hip-odoo/odoo,hip-odoo/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,dfang/odoo,ygol/odoo,bplancher/odoo,bplancher/odoo,ygol/odoo,storm-computers/odoo,ygol/odoo,Elico-Corp/odoo_OCB,ygol/odoo,laslabs/odoo,Elico-Corp/odoo_OCB