Change Fs.Put so that it must cope with existing files

This should fix duplicate files on drive and 409 errors on
amazonclouddrive however it will slow down the upload slightly as
another roundtrip will be needed.

None of the other Fses needed adjusting.

Fixes #483

amazonclouddrive/amazonclouddrive.go

@@ -425,6 +425,17 @@ func (f *Fs) Put(in io.Reader, src fs.ObjectInfo) (fs.Object, error) {
 		fs:     f,
 		remote: remote,
 	}
+	// Check if object already exists
+	err := o.readMetaData()
+	switch err {
+	case nil:
+		return o, o.Update(in, src)
+	case fs.ErrorDirNotFound, acd.ErrorNodeNotFound:
+		// Not found so create it
+	default:
+		return nil, err
+	}
+	// If not create it
 	leaf, directoryID, err := f.dirCache.FindPath(remote, true)
 	if err != nil {
 		return nil, err