Add missing API Blob.safeClose()

polyfill/Blob.js

     // Blob data from file path
     else if(typeof data === 'string' && data.startsWith('RNFetchBlob-file://')) {
       log.verbose('create Blob cache file from file path', data)
+      // set this flag so that we know this blob is a wrapper of an existing file
+      this._isReference = true
       this._ref = String(data).replace('RNFetchBlob-file://', '')
       let orgPath = this._ref
       if(defer)
     })
   }
 
+  safeClose() {
+    if(this._closed)
+      return Promise.reject('Blob has been released.)
+    this._closed = true
+    if(!this._isReference) {
+      return fs.unlink(this._ref).catch((err) => {
+        console.warn(err)
+      })   
+    }
+    else {
+      return Promise.resolve()
+    }
+  }
+
   _invokeOnCreateEvent() {
     log.verbose('invoke create event', this._onCreated)
     this._blobCreated = true