Add global replace for processing raw headers

fetch.js

@@ -360,7 +360,7 @@
     var headers = new Headers()
     // 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 ]+/, ' ')
+    var preProcessedHeaders = rawHeaders.replace(/\r?\n[\t ]+/g, ' ')
     preProcessedHeaders.split(/\r?\n/).forEach(function(line) {
       var parts = line.split(':')
       var key = parts.shift().trim()