fix: ignore 'content-type' header capitalization (sveltejs#1463)

Fixes sveltejs#1463

.changeset/pretty-cycles-learn.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Allow non-lowercase 'content-type' header in ssr fetch requests

packages/kit/src/runtime/server/parse_body/index.js

@@ -1,11 +1,13 @@
+import { lowercase_keys } from '../utils.js';
 import { read_only_form_data } from './read_only_form_data.js';
 
 /** @param {import('types/hooks').Incoming} req */
 export function parse_body(req) {
 	const raw = req.rawBody;
 	if (!raw) return raw;
 
-	const [type, ...directives] = req.headers['content-type'].split(/;\s*/);
+	const headers = lowercase_keys(req.headers);
+	const [type, ...directives] = headers['content-type'].split(/;\s*/);
 
 	if (typeof raw === 'string') {
 		switch (type) {

packages/kit/src/runtime/server/parse_body/parse_body.spec.js

@@ -0,0 +1,21 @@
+import { test } from 'uvu';
+import * as assert from 'uvu/assert';
+import { parse_body } from './index.js';
+
+test('parses a body with a lowercase content-type', () => {
+	const msg = {
+		rawBody: JSON.stringify('the body'),
+		headers: { 'content-type': 'application/json' }
+	};
+	assert.equal(parse_body(msg), 'the body');
+});
+
+test('parses a body with a non-lowercase content-type', () => {
+	const msg = {
+		rawBody: JSON.stringify('the body'),
+		headers: { 'Content-Type': 'application/json' }
+	};
+	assert.equal(parse_body(msg), 'the body');
+});
+
+test.run();