fix: join multiple header values by comma, not semicolon (#49)

* fix(flattenHeadersObject): join multiple values by comma

* fix(flattenHeadersList): join multiple values by comma

* docs: update readme

README.md

@@ -208,7 +208,7 @@ appendHeader(
 import { flattenHeadersList } from 'headers-polyfill'
 
 flattenHeadersList([['content-type', ['text/plain', 'image/png']]])
-// ['content-type', 'text/plain; image/png']
+// ['content-type', 'text/plain, image/png']
 ```
 
 - `flattenHeadersObject: (o: Record<string, string | string[]>): Record<string, string>`
@@ -219,5 +219,5 @@ import { flattenHeadersObject } from 'headers-polyfill'
 flattenHeadersObject({
   'content-type': ['text/plain', 'image/png'],
 })
-// { 'content-type': 'text/plain; image/png' }
+// { 'content-type': 'text/plain, image/png' }
 ```

src/transformers/flattenHeadersList.test.ts

@@ -9,7 +9,7 @@ describe('given a headers list', () => {
     ]
 
     expect(flattenHeadersList(headersList)).toEqual([
-      ['accept', 'application/json; text/xml'],
+      ['accept', 'application/json, text/xml'],
       ['content-type', 'application/pdf'],
     ])
   })

src/transformers/flattenHeadersList.ts

@@ -2,6 +2,6 @@ import { HeadersList, FlatHeadersList } from '../glossary'
 
 export function flattenHeadersList(list: HeadersList): FlatHeadersList {
   return list.map(([name, values]) => {
-    return [name, ([] as string[]).concat(values).join('; ')]
+    return [name, ([] as string[]).concat(values).join(', ')]
   })
 }

src/transformers/flattenHeadersObject.test.ts

@@ -9,7 +9,7 @@ describe('given a headers object', () => {
     }
 
     expect(flattenHeadersObject(headersObject)).toEqual({
-      Accept: 'application/json; text/xml',
+      Accept: 'application/json, text/xml',
       'Content-Type': 'application/pdf',
     })
   })

src/transformers/flattenHeadersObject.ts

@@ -7,7 +7,7 @@ export function flattenHeadersObject(
   return reduceHeadersObject<FlatHeadersObject>(
     headersObject,
     (headers, name, value) => {
-      headers[name] = ([] as string[]).concat(value).join('; ')
+      headers[name] = ([] as string[]).concat(value).join(', ')
       return headers
     },
     {}