Update gettext-parser to v4; adjust code & docs accordingly

Author: eemeli

README.md

@@ -47,7 +47,7 @@ msgstr "Nema zvezde po imenu %(starname)s."
 `)
 
 const MessageFormat = require('messageformat')
-const mf = new MessageFormat({ [headers.language]: pluralFunction })
+const mf = new MessageFormat({ [headers.Language]: pluralFunction })
 const messages = mf.compile(translations)
 
 messages['Time: %1 second']([1])
@@ -98,7 +98,7 @@ For more options, take a look at the [source](./index.js).
 Both functions return an object containing the following fields:
 
 - `headers` (object) – The raw contents of the input file's headers, with keys
-  lower-cased
+  using canonical casing.
 - `pluralFunction` (function) – An appropriate pluralisation function to use for
   the output translations, suitable to be used directly by [messageformat]. May
   be `null` if none was set in `options` and if the input did not include a

index.js

@@ -69,7 +69,7 @@ const convert = (parse, input, options) => {
   options = Object.assign({}, defaultOptions, options)
   const { headers, translations } = parse(input, options.defaultCharset)
   if (!options.pluralFunction) {
-    options.pluralFunction = getPluralFunction(headers['plural-forms'])
+    options.pluralFunction = getPluralFunction(headers['Plural-Forms'])
   }
   let hasContext = false
   for (const context in translations) {

package-lock.json

@@ -17,7 +17,7 @@
     "node": ">=6.0"
   },
   "dependencies": {
-    "gettext-parser": "^1.4.0"
+    "gettext-parser": "^4.0.4"
   },
   "devDependencies": {
     "messageformat": "^2.3.0"