[Filesystem] Changed the mode for a target file in copy() to be write only

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9789
| License       | MIT
| Doc PR        | -

Stream wrappers like S3 [do not support w+](http://docs.aws.amazon.com/aws-sdk-php/latest/class-Aws.S3.StreamWrapper.html) and we don't read here anyway (as pointed out in #9789).