Allow Request Headers to be set via config (#1464)

Fixes #1463.

mithril.d.ts

@@ -840,6 +840,11 @@ declare namespace Mithril {
 		*/
 		password?: string;
 
+		/**
+		* An optional object map of headers with their respective values
+		*/
+		headers?: {[key: string]: string};
+
 		/**
 		* An optional function to run between `open` and `send`, useful for
 		* adding request headers or using XHR2 features such as the `upload`

mithril.js

@@ -2171,6 +2171,14 @@
 			if (maybeXhr != null) xhr = maybeXhr
 		}
 
+		if (isObject(options.headers)) {
+			for (var header in options.headers) {
+				if (hasOwn.call(options.headers, header)) {
+					xhr.setRequestHeader(header, options.headers[header])
+				}
+			}
+		}
+
 		var data = options.method === "GET" || !options.data ? "" : options.data
 
 		if (data && !isString(data) && data.constructor !== global.FormData) {

test/mithril.request.js

@@ -146,6 +146,32 @@ describe("m.request()", function () {
 		expect(xhr.$headers).to.not.have.property("Content-Type")
 	})
 
+
+	it("sets xhr request headers as per the headers config", function () {
+		var error = m.prop()
+
+		m.request({
+			method: "POST",
+			url: "test",
+			headers: {
+				"Authorization" : "Bearer 12345abcd12345",
+				"CustomHeader" : "CustomValue"
+			}
+		}).then(null, error)
+
+		var xhr = mock.XMLHttpRequest.$instances.pop()
+		xhr.onreadystatechange()
+
+		expect(xhr.$headers).to.have.property(
+			"Authorization",
+			"Bearer 12345abcd12345")
+
+		expect(xhr.$headers).to.have.property(
+			"CustomHeader",
+			"CustomValue")
+	})
+
+
 	it("correctly sets initial value", function () {
 		var prop = m.request({
 			method: "POST",