ArangoHost#toString() should return a human-readable representation in the form host:port

[antonsarov]

Branches 2.7, 3.0 and 3.1 use the ArangoHost class which includes a host and a port. There is an agreed-upon human-readable representation in the form "host:port". This is also defined in RFC 3986. The toString() method should rely on this.

[mvollmary]

Pull request accepted in branch 3.1. Commit also merged in branch 2.7 and 3.0.
Changes will be in the next bugfix release.

Thanks a lot
Mark