Add context path to the redirect URL in saml_login.jsp

[max-allan-surevine]

NB I am no java dev and no virtually nothing about Confluence's internal workings, so this may be an awful way to make it work!
I was using confluence with a context path and noticed that logins worked but redirected me to a URL without the context path. Looking at packets shows :
GET /confluence/display/SIC/Hello HTTP/1.1
gets a 302 redirect to
Location: /confluence/login.action?os_destination=%2Fpages%2Fviewpage.action%3FspaceKey%3DSIC%26title%3DHello&permissionViolation=true
With the default authenticator.

However, with the SAML plugin, redirect to :
Location: /confluence/saml_login.jsp?os_destination=%2Fpages%2Fviewpage.action%3FspaceKey%3DSIC%26title%3DHello

And the redirect uri includes the os_destination but not the context path.
My simple fix could maybe do with some logic if context path is "" but I think adding "" to the rest of the path is probably safe enough. I don't know if the os_destination should ever include the context. I couldn't find any docs about what it is supposed to be.