Force webservice rewrite to "api/" with the slash

The "?" after ^api/ makes the slash optional (? = 0 or 1 time for the last group or char). If we use a friendly url which begins with api, the visitor gets redirected to the webservice, and never gets the page. Try it for example with  the friendly url "apiculture" (beekeeping :-) ).
Merge pull request #2025 from zimmi1/patch-5

[*] CORE : Force webservice rewrite to "api/" with the slash