Initialise Headers map to avoid runtime panic when purging.

[Integralist]

Problem: #266

The call to new(RequestOptions) initialises the struct with zero values. So the RequestOptions.Headers map is nil by default . Hence trying to assign a value to nil results in panic: assignment to entry in nil map:

Example code that validated this:

p, err := client.Purge(&fastly.PurgeInput{
	URL:  "http://testing-gofastly-purge.integralist-test.com.global.prod.fastly.net/anything/foo",
	Soft: true,
})

[kailan]

Looks good 👍