auto-send Content-Type header if using JSON.stringify as serialize in m.request

mithril.js

@@ -561,6 +561,9 @@ Mithril = m = new function app(window) {
 				else options.onerror({type: "error", target: xhr})
 			}
 		}
+		if (options.serialize == JSON.stringify && options.method != "GET") {
+			xhr.setRequestHeader("Content-Type", "application/json; charset=utf-8");
+		}
 		if (typeof options.config == "function") {
 			var maybeXhr = options.config(xhr, options)
 			if (maybeXhr !== undefined) xhr = maybeXhr
@@ -592,8 +595,8 @@ Mithril = m = new function app(window) {
 	m.request = function(xhrOptions) {
 		if (xhrOptions.background !== true) m.startComputation()
 		var deferred = m.deferred()
-		var serialize = xhrOptions.serialize || JSON.stringify
+		var serialize = xhrOptions.serialize = xhrOptions.serialize || JSON.stringify
-		var deserialize = xhrOptions.deserialize || JSON.parse
+		var deserialize = xhrOptions.deserialize = xhrOptions.deserialize || JSON.parse
 		var extract = xhrOptions.extract || function(xhr) {
 			return xhr.responseText.length === 0 && deserialize === JSON.parse ? null : xhr.responseText
 		}

tests/mithril-tests.js

@@ -1211,6 +1211,13 @@ function testMithril(mock) {
 		m.endComputation()
 		return prop() === undefined && error() === "no error" && exception.message == "error occurred"
 	})
+	test(function() {
+		var error = m.prop("no error")
+		var prop = m.request({method: "POST", url: "test"}).then(null, error)
+		var xhr = mock.XMLHttpRequest.$instances.pop()
+		xhr.onreadystatechange()
+		return xhr.$headers["Content-Type"] == "application/json; charset=utf-8"
+	})
 
 	//m.deferred
 	test(function() {

tests/mock.js

@@ -85,6 +85,10 @@ mock.window = new function() {
 	}
 	window.XMLHttpRequest = new function() {
 		var request = function() {
+			this.$headers = {}
+			this.setRequestHeader = function(key, value) {
+				this.$headers[key] = value
+			}
 			this.open = function(method, url) {
 				this.method = method
 				this.url = url