Transform user set headers to lowercase (#613)

* Change test to use header got wants to set

* Lowercase headers set by user

package.json

@@ -39,6 +39,7 @@
 		"decompress-response": "^3.3.0",
 		"duplexer3": "^0.1.4",
 		"get-stream": "^4.0.0",
+		"lowercase-keys": "^1.0.1",
 		"mimic-response": "^1.0.1",
 		"p-cancelable": "^0.5.0",
 		"to-readable-stream": "^1.0.0",

source/normalize-arguments.js

@@ -3,6 +3,7 @@ const {URL, URLSearchParams} = require('url'); // TODO: Use the `URL` global whe
 const is = require('@sindresorhus/is');
 const toReadableStream = require('to-readable-stream');
 const urlParseLax = require('url-parse-lax');
+const lowercaseKeys = require('lowercase-keys');
 const isRetryOnNetworkErrorAllowed = require('./is-retry-on-network-error-allowed');
 const urlToOptions = require('./url-to-options');
 const isFormData = require('./is-form-data');
@@ -15,10 +16,15 @@ const retryAfterStatusCodes = new Set([413, 429, 503]);
 // While `normalize` does `preNormalize` + handles things related to dynamic options, like URL, headers, body, etc.
 const preNormalize = options => {
 	options = {
-		headers: {},
 		...options
 	};
 
+	if (is.nullOrUndefined(options.headers)) {
+		options.headers = {};
+	} else {
+		options.headers = lowercaseKeys(options.headers);
+	}
+
 	if (options.baseUrl && !options.baseUrl.toString().endsWith('/')) {
 		options.baseUrl += '/';
 	}

test/headers.js

@@ -73,11 +73,11 @@ test('host', async t => {
 test('transform names to lowercase', async t => {
 	const headers = (await got(s.url, {
 		headers: {
-			'USER-AGENT': 'test'
+			'ACCEPT-ENCODING': 'identity'
 		},
 		json: true
 	})).body;
-	t.is(headers['user-agent'], 'test');
+	t.is(headers['accept-encoding'], 'identity');
 });
 
 test('setting content-length to 0', async t => {