Support obs-fold as header delimiter

fetch.js

@@ -358,7 +358,10 @@
 
   function parseHeaders(rawHeaders) {
     var headers = new Headers()
-    rawHeaders.split(/\r?\n/).forEach(function(line) {
+    // Replace instances of \r\n and \n followed by at least one space or horizontal tab with a space
+    // https://tools.ietf.org/html/rfc7230#section-3.2
+    var preProcessedHeaders = rawHeaders.replace(/\r?\n[\t ]+/, ' ')
+    preProcessedHeaders.split(/\r?\n/).forEach(function(line) {
       var parts = line.split(':')
       var key = parts.shift().trim()
       if (key) {