Split Content-Type extraction to Request and Body

It is done in this way in the spec.

src/body.js

@@ -249,5 +249,25 @@ export function clone(instance) {
 	return body;
 }
 
+/**
+ * Performs the operation "extract a `Content-Type` value from |object|" as
+ * specified in the specification:
+ * https://fetch.spec.whatwg.org/#concept-bodyinit-extract
+ *
+ * This function assumes that instance.body is present and non-null.
+ *
+ * @param   Mixed  instance  Response or Request instance
+ */
+export function extractContentType(instance) {
+	// detect form data input from form-data module
+	if (typeof instance.body.getBoundary === 'function') {
+		return `multipart/form-data;boundary=${instance.body.getBoundary()}`;
+	}
+
+	if (typeof instance.body === 'string') {
+		return 'text/plain;charset=UTF-8';
+	}
+}
+
 // expose Promise
 Body.Promise = global.Promise;

src/index.js

@@ -68,11 +68,6 @@ function fetch(url, opts) {
 			headers.set('accept', '*/*');
 		}
 
-		// detect form data input from form-data module, this hack avoid the need to pass multipart header manually
-		if (!headers.has('content-type') && options.body && typeof options.body.getBoundary === 'function') {
-			headers.set('content-type', `multipart/form-data; boundary=${options.body.getBoundary()}`);
-		}
-
 		// bring node-fetch closer to browser behavior by setting content-length automatically
 		if (!headers.has('content-length') && /post|put|patch|delete/i.test(options.method)) {
 			if (typeof options.body === 'string') {

src/request.js

@@ -7,7 +7,7 @@
 
 import { format as format_url, parse as parse_url } from 'url';
 import Headers from './headers.js';
-import Body, { clone } from './body';
+import Body, { clone, extractContentType } from './body';
 
 /**
  * Request class
@@ -46,6 +46,13 @@ export default class Request extends Body {
 		this.redirect = init.redirect || input.redirect || 'follow';
 		this.headers = new Headers(init.headers || input.headers || {});
 
+		if (init.body) {
+			const contentType = extractContentType(this);
+			if (contentType && !this.headers.has('Content-Type')) {
+				this.headers.append('Content-Type', contentType);
+			}
+		}
+
 		// server only options
 		this.follow = init.follow !== undefined ?
 			init.follow : input.follow !== undefined ?