Implement Blob.close().

LayoutTests/http/tests/xmlhttprequest/post-blob-content-type-tests.js

 var xhrBlobTestUrl = '/xmlhttprequest/resources/access-control-allow-lists.php';
 
 var xhrBlobTestCases = [{
     mime: 'text/plain;charset=utf-8',
     expectedMime: 'text/plain;charset=utf-8'
 }, {
     mime: 'Invalid/Type\r\n;charset=koi8-r',
     expectedMime: ''
 }, {
     mime: 'ASCII/CR\r;charset=invalid',
@@ skipping 13 matching lines @@
 }, {
     mime: 'multipart/mixed;boundary="--blob-boundary',
     expectedMime: ''
 }, {
     mime: 'multipart/mixed;boundary="--blob-boundary"',
     expectedMime: 'multipart/mixed;boundary="--blob-boundary"'
 }, {
     mime: '',
     expectedMime: ''
 }, {
+    mime: '',
+    expectedMime: '',
+    closeBeforeSend: true
+}, {
     mime: '\u0422\u0435\u0441\u0442',
     expectedMime: '',
     shouldThrow: true
 }, {
     mime: 'text/plain;charset=utf-8',
     expectedMime: 'text/plain;charset=utf-8',
     shouldThrow: true,
     url: 'http://localhost:8000',
     allowOrigin: ''
 }, {
@@ skipping 15 matching lines @@
     file: 'hello.txt'
 }, {
     mime: 'multipart/mixed;boundary="--blob-boundary"',
     expectedMime: 'multipart/mixed;boundary="--blob-boundary"',
     file: 'hello.txt'
 }, {
     mime: '\u0422\u0435\u0441\u0442',
     expectedMime: '',
     shouldThrow: true,
     file: 'hello.txt'
+}, {
+    mime: '',
+    expectedMime: '',
+    file: 'hello.txt',
+    closeBeforeSend: true
 }];