generates bad URL without trailing slash

Hi.
first details about my WP.
version 3.3.1.
installed in a subdirectory like : https://www.mysite.com/blog/
Blog was running super fine with previous theme ( my child theme built on Thematic) until….

I installed (purchased ) a new theme ( first time…but in a hurry). Theme looks ok, works for people and was recently released.
BUT :
I am testing it and get a problem with the logo URL and the trailing SLASH / that does not appear at the end of the URL of the logo…. Resulting into an endless loop when I click on the header logo which has the unfortunate URL :
https://www.mysite.com/blog notice the absence of trailing / at the end.
The problem is that when I use any other theme ( I’ve tried Twenty eleven for i.e) logo URL works fine and gives the trailing slash at the end :
https://www.mysite.com/blog/

I’ve contacted the theme designer and he told me that the theme automatically grab the site url of my general settings. Which I understand.
BTW : My general settings are ok ( never changed them) and have of course the site URL without trailing slash at the end as WP 3.3.1 does not allow to add it.
Question : why some theme correctly add the trailing slash at the end of the LOGO url and why other don’t?
second question : The theme function that generates the URL is
echo '<a href="'.get_bloginfo('url').'">…Is there anyway to force WP to put this / at the end ?
Any help will be loved…I am battling with this for 4 days…