Fix ts_config_http:parse_URL/4 to deal with empty paths

[tisba]

This is a fix to address #227

For URLs with an empty path, like http://example.com?foo=bar, we used to see this when using a dynamic URL for making a HTTP request (as reported by @adrian7):

=INFO REPORT==== 16-Jun-2018::19:10:21 ===
             ts_http:(6:<0.158.0>) URL dynamic subst: {url,http,
                                                       "example.com?foo=bar",
                                                       undefined,"/",[]}

It turns out ts_config_http:parse_URL/4 does not stop at ? when trying to identify the host in a given absolute URL.

With this PR, ts_config_http:parse_URL/4 now properly detects that the host part has been finished (because we encounter the ?) and produces an empty path instead. We have to set URL#url{path="/"}; because otherwise we will produce an invalid HTTP request like: GET ?foo=bar HTTP/1.1

=INFO REPORT==== 16-Jun-2018::19:07:20 ===
             ts_http:(6:<0.158.0>) URL dynamic subst: {url,http,
                                                       "example.com",
                                                       undefined,"/",
                                                       "foo=bar"}

[tisba]

If you could have a quick look @nniclausse, this would be really nice. The change is rather simple and I currently don't see any potential issues arising from this, but maybe you can have a quick look nonetheless?

[nniclausse]

test case was missing; added. looks good for me !

[tisba]

duh, still need to get more familiar with testing in Erlang/tsung. Thanks @nniclausse!