Fix Headers iterable initializer handling

src/headers.js

@@ -44,10 +44,14 @@ export default class Headers {
 		      this.append(name, value);
 		    }
 		  }
-		} else if (Array.isArray(headers)) {
+		} else if (typeof headers === 'object' && headers[Symbol.iterator]) {
 			// array of tuples
 			for (let el of headers) {
-				if (!Array.isArray(el) || el.length !== 2) {
+				if (typeof el !== 'object' || !el[Symbol.iterator]) {
+					throw new TypeError('Header pairs must be an iterable object');
+				}
+				el = Array.from(el);
+				if (el.length !== 2) {
 					throw new TypeError('Header pairs must contain exactly two items');
 				}
 				this.append(el[0], el[1]);
@@ -59,6 +63,8 @@ export default class Headers {
 				// will handle it.
 				this.append(prop, headers[prop]);
 			}
+		} else if (headers != null) {
+			throw new TypeError('Provided initializer must be an object');
 		}
 
 		Object.defineProperty(this, Symbol.toStringTag, {

test/test.js

@@ -1435,19 +1435,37 @@ describe(`node-fetch with FOLLOW_SPEC = ${defaultFollowSpec}`, () => {
 		expect(h3Raw['b']).to.include('1');
 	});
 
-	it('should accept headers as an array of tuples', function() {
-		const headers = new Headers([
+	it('should accept headers as an iterable of tuples', function() {
+		let headers;
+
+		headers = new Headers([
 			['a', '1'],
 			['b', '2'],
 			['a', '3']
 		]);
 		expect(headers.getAll('a')).to.deep.equal(['1', '3']);
 		expect(headers.getAll('b')).to.deep.equal(['2']);
+
+		headers = new Headers([
+			new Set(['a', '1']),
+			['b', '2'],
+			new Map([['a', null], ['3', null]]).keys()
+		]);
+		expect(headers.getAll('a')).to.deep.equal(['1', '3']);
+		expect(headers.getAll('b')).to.deep.equal(['2']);
+
+		headers = new Headers(new Map([
+			['a', '1'],
+			['b', '2']
+		]));
+		expect(headers.getAll('a')).to.deep.equal(['1']);
+		expect(headers.getAll('b')).to.deep.equal(['2']);
 	});
 
 	it('should throw a TypeError if non-tuple exists in a headers initializer', function() {
 		expect(() => new Headers([ ['b', '2', 'huh?'] ])).to.throw(TypeError);
 		expect(() => new Headers([ 'b2' ])).to.throw(TypeError);
+		expect(() => new Headers('b2')).to.throw(TypeError);
 	});
 
 	it('should support fetch with Request instance', function() {