We have found a workaround for this, where you add the source=webclient parameter, crucially, to the END of the twitter address. When you do this, Twitter no longer adds the trailing slash. However, given how Twitter like to change their formats and codes around, there's no guarantee this will work forever :-)
For example...
<a href="https://twitter.com/intent/tweet?text=http://www.rsc.org%2FEducation%2FEiC%2Fissues%2F2013January%2F50th%2Delement%2Dtin%2Easp&source=webclient" target="_blank">Twitter this</a>