add XML support

[tmm1]

cc #78

[earthboundkid]

I am pretty happy with this, but I think in 2023 XML is sort of niche, so it doesn't need to be a pair of methods on Builder, and it could just be functions instead. Builder already has so many methods that it junks up the docs. Also I'm not sure about it, but I think if it's a method then you have to import encoding/xml along with this package, but if it's not maybe it gets tree shaken out? Might want to experiment with it to figure that out.

Also, it's nice to have an example test. Is there a good public API that uses XML to demo?

[tmm1]

Hmm, I am definitely using the builder methods. Having to set application/xml each time manually would be a pain without BodyXML.

[tmm1]

Also, it's nice to have an example test. Is there a good public API that uses XML to demo?

I looked for an xmlplaceholder api but didn't see one pop up right away.

[earthboundkid]

Having to set application/xml each time manually would be a pain without BodyXML.

Something I wonder about for BodyJSON is if people are inconvenienced when they want to set a different content type than application/json. For XML, I bet there are servers that require the content type to be application/xml+myapp.v3 or whatever. Maybe there should be some way to signal what the default content type for a BodyGetter is while still letting you override it. Not sure how it would work though.

[earthboundkid]

I did a quick test of putting a function that imports encoding/xml next to another that doesn't and then not using the xml function, and it ends up not being tree shaken out. So, if we add this, everyone's build will end up including encoding/xml even if they don't use it. Probably not a big deal in practice, but some people want smaller binaries.

What if we move these into a subfolder, maybe reqxml.Body(contentType string, obj any) requests.Config and reqxml.To(any) requests.Handler? Body can default to using application/xml as the content type when it's an empty string.

Use would look like

err := requests.
  URL("http://whatever.com").
  Config(reqxml.Body("", req)).
  Handler(reqxml.To(&res)).
  Fetch(ctx)

[earthboundkid]

Here is a funny XML example:

func ExampleBuilder_ToXML() {
	type CD struct {
		Title   string  `xml:"TITLE"`
		Artist  string  `xml:"ARTIST"`
		Country string  `xml:"COUNTRY"`
		Price   float64 `xml:"PRICE"`
		Year    int     `xml:"YEAR"`
	}
	type Catalog struct {
		CDs []CD `xml:"CD"`
	}
	var cat Catalog
	err := requests.
		URL("https://www.w3schools.com/xml/cd_catalog.xml").
		ToXML(&cat).
		Fetch(context.Background())
	if err != nil {
		fmt.Println("Error!", err)
	}
	for _, cd := range cat.CDs {
		if cd.Price > 10 && cd.Year < 1990 {
			fmt.Printf("%s - %s $%.2f", cd.Artist, cd.Title, cd.Price)
		}
	}
	// Output:
	// Bob Dylan - Empire Burlesque $10.90
}

[earthboundkid]

I rolled your commit into #82. Let me know if it looks good to you.

[earthboundkid]

Merged in #82.