cp/mirror: Use proper paths for windows during copyObject.

[harshavardhana]

Fixes #1857

[codecov-io]

Current coverage is 8.65% (diff: 0.00%)

Merging #1897 into master will decrease coverage by <.01%

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[krisis]

LGTM, tested locally on a windows laptop and mc cp works as expected with this change.

[harshavardhana]

This fix is complete for the present form of path handling, what we need to do here is fix the newURL() to always provide paths with "/" not with "\" in them. Currently i see that mc requires an overhaul in terms of path handling across the board which will do subsequently after this merge. Such a change needs to proceed before we bring in mc admin capabilities.