provide default arguments for http_build_query in all cases

I was running into a problem with flickrapi using oauth_connect's DrupalOAuthClient get function - there's a place where it uses http_build_query without providing explicit options and arg_separator was getting a bad default (& instead of &) - this caused requests to be malformed.

I'll attach a patch that provides a default set of arguments to http_build_query as used in other places in this module.