Allow http.Client used in discovery to be modified (typically for sec…

[steve-hellwege-wdc]

…urity reasons)

[why] The importing application may have some specific security requirements that necessitate a change to the http.Client or http.Transport used when fetching the xml from the UPnp server. For example, the importing application may want to restrict localhost calls which could be made by an attack server on the local network.

[how] Create a global HTTPClient which defaults to http.DefaultClient. This allows the importing application to modify this global if it wishes to make changes to the http.Client/http.Transport used when fetching the xml from the UPnP server.

[steve-hellwege-wdc]

@huin ,can you please review/merge this? Thank you.

[huin]

I'll take a look on the weekend, my weeknights are a bit busy.

[huin]

Thanks!