Merge pull request robinhood-unofficial#91 from jasonleehodges/bug/in…

…struments-endpoint

BUG FIX: instruments endpoint

Robinhood/endpoints.py

@@ -33,7 +33,7 @@ def instruments(instrumentId=None, option=None):
     Return information about a specific instrument by providing its instrument id. 
     Add extra options for additional information such as "popularity"
     '''
-    return "https://api.robinhood.com/instruments/{id}/".format(id=instrumentId) + "{_option}/".format(_option=option) if option else ""
+    return "https://api.robinhood.com/instruments/" + ("{id}/".format(id=instrumentId) if instrumentId else "") + ("{_option}/".format(_option=option) if option else "")
 
 def margin_upgrades():
     return "https://api.robinhood.com/margin/upgrades/"