eliminate double-slash when api_version is empty string

Double-slashes can occur when the API version is the empty string,
e.g. identi.ca.  URI.parse raises an exception when its input starts
with a double-slash and contains underscore, e.g.
'//direct_messages.json?count=1'

Strip double-slashes when merging the api_version with the given
path to evade this exception.