Add support for download xhr headers (#375)

papaparse.js

@@ -581,6 +581,16 @@
 			}
 
 			xhr.open('GET', this._input, !IS_WORKER);
+			// Headers can only be set when once the request state is OPENED
+			if (this._config.downloadRequestHeaders)
+			{
+				var headers = this._config.downloadRequestHeaders;
+
+				for (var headerName in headers)
+				{
+					xhr.setRequestHeader(headerName, headers[headerName]);
+				}
+			}
 
 			if (this._config.chunkSize)
 			{