Amend short form of French months (continuing #64)

The Firefox French language pack contains some mistakes regarding
the short form of months. As a consequence, French month parsing
didn't work.

Please note that these values aren't even the correct abbreviations
but only the three or four first letters of the correct abbreviations.
See the French CSL locale for the correct abbreviations.

resource/schema/dateFormats.json

@@ -306,13 +306,13 @@
 			"mar",
 			"avr",
 			"mai",
-			"jun",
-			"jul",
+			"juin",
+			"juil",
 			"aoû",
 			"sep",
 			"oct",
 			"nov",
-			"dec"
+			"déc"
 		],
 		"long": [
 			"janvier",

test/tests/dateTest.js

@@ -9,7 +9,7 @@ describe("Zotero.Date", function() {
 			"October", "November", "December"
 		];
 		var frenchShort = [
-			"jan", "fév", "mar", "avr", "mai", "jun", "jul", "aoû", "sep", "oct", "nov", "dec"
+			"jan", "fév", "mar", "avr", "mai", "juin", "juil", "aoû", "sep", "oct", "nov", "déc"
 		];
 		var frenchLong = [
 			"janvier", "février", "mars", "avril", "mai", "juin", "juillet", "août", "septembre",