fix: correctly separate multiple scopes with '&'

[edwardgeorge]

was generating invalid query strings with multiple scopes, eg:

.authenticate(&["repository:foo:pull", "repository:bar:pull"])

is serialised as:

?service=...&scope=repository:foo:pullscope=repository:bar:pull

note the missing '&' between pull and scope.

[lucab]

@edwardgeorge good catch! If you don't mind, can you please add unit tests for the following three cases:

• no scopes
• one scope
• more than one scope

[edwardgeorge]

@lucab sure, no problem.

[edwardgeorge]

@lucab updated with three tests

[edwardgeorge]

@steveeJ updated with consolidated, parametrized tests