Set PHP's user_agent if empty

[BitPoet]

Set PHP's user_agent to a non-empty value if not set, as github api requests with an empty/missing user-agent header are rejected with status 403 (forbidden). This should fix issue #11.

[matjazpotocnik]

@BitPoet Hi! Do you think this is still needed? That is, if a GitHub request with an empty/missing user-agent header is rejected? How do I check this? I never got any errors or warnings using this module.

[BitPoet]

I'm not sure how to check this, as the error has always shown up inconsistently, but it probably should still be dealt with. The GitHub API docs specifically require a value in the User-Agent header.

[ryancramerdesign]

@BitPoet @matjazpotocnik This module uses WireHttp so the user-agent header should always be present. WireHttp supplies a default one if any outbound http request fails to specify one. Though it looks like this module uses its name (ProcessWireUpgrade) as the user-agent header.