data-source/http: Ensure HTTP request body is not sent unless configured

[bflad]

Closes #388

Previously the HTTP request would unexpectedly always contain a body for all requests. Certain HTTP server implementations are sensitive to this data existing if it is not expected. Requests now only contain a request body if the request_body attribute is explicitly set. To exactly preserve the previous behavior, set request_body = "".

Added new acceptance testing for request_body handling. Go net/http server implementations seem to ignore this problematic situation (or it was not easy to determine), so also verified with a real world configuration using a temporary acceptance test as well as manually running Terraform using a development build containing these changes:

// Reference: https://github.com/hashicorp/terraform-provider-http/issues/388
func TestDataSource_RequestBody_Null(t *testing.T) {
	t.Parallel()

	resource.Test(t, resource.TestCase{
		ProtoV5ProviderFactories: protoV5ProviderFactories(),
		Steps: []resource.TestStep{
			{
				Config: `
					data "http" "test" {
						url = "https://www.cloudflare.com/ips-v4"
					}`,
				Check: resource.ComposeAggregateTestCheckFunc(
					resource.TestCheckResourceAttr("data.http.test", "status_code", "200"),
				),
			},
		},
	})
}

This real world acceptance test is not added as the server may change its implementation in the future.

[austinvalle]

LGTM 🚀

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