Add basic auth support: Sling.SetBasicAuth()

[AaronO]

Sling is one of the nicest libraries I found, it's composable and elegant. However I found it frustrating that it lacked Basic Auth support which is a must have when writing API clients, so I thought I would add here.

Tried to keep it as simple as possible and follow existing conventions (allow chaining etc ...)

Let me know if you have any questions.

[dghubble]

// SetBasicAuth sets the Authorization header to use HTTP Basic Authentication
// with the provided username and password. With HTTP Basic Authentication
// the provided username and password are not encrypted.

^ let's match net/http so its implicitly clear it does the same.

[dghubble]

Simple enough to do it directly on the http.Request but I can see the desire to use the same chaining (fluent stlye) for this with Sling. Can you add a sanity test with the user: Aladdin, password: "open sesame" example from the RFC please?

Otherwise, looks good for merge. Thanks for the PR.

[AaronO]

@dghubble Voilà, updated comments and added tests.

You can indeed do it on the http.Request but that doesn't feel elegant and is especially awkward when building an API client. Now you can get a full base client that's chainable in one line:

baseClient := sling.New().Base("https://api.example.com").SetBasicAuth("admin", "secret")

So you can set your auth once when building your base client, then forget about it which is quite nice.

Since sling is especially built for interacting with APIs it seemed odd that adding auth (required for all production APIs) was awkward or didn't follow the simplicity and ease of use elsewhere (automatic JSON serialization, etc ...)

[AaronO]

@dghubble Sorry for taking a few days to reply to your feedback. I got swamped with work. Let me know if you have any further questions.

[dghubble]

Squash merged as c1694aa. Thanks!