Use a copy of the files when signing with a memorySigner

[rwngallego]

I've found that when using a memorySigner, the original files from the template get corrupted in subsequent calls to the CreateSignedAndZippedPersonalizedPassArchive method. I think that similar to the fileSigner implementation, the memorySigner one should work with a temporary copy of the template files and not affect the original ones. Once the pass is signed and compressed, the temporary copy should be removed, as the fileSigner does.

Given that in Go the maps are passed by reference and not by value, the CreateSignedAndZippedPersonalizedPassArchive method is propagating the manifest.json and signature files calculated in a first call, to the subsequent calls. For example, in a first call, it correctly generates the following manifest.json file and a valid Apple Wallet pass:

{
    "footer.png": "ec95fabe4c282cf0c5acdbc3d58d2adeb2847385",
    "icon.png": "cf6a2cd30798d472f0cb81364d25ba6e895b0cae",
    "[email protected]": "3915ac1b648893731acdd6f53c28cf1179bb72bc",
    "logo.png": "0b9c6f6022ef1031d031cae2f7541173d9039504",
    "pass.json": "cc7095122351b0abdc6724bf4d0dc8a2b582a799"
}

However, In a second call, the manifest.json file would have these extra entries that make the generated pass to be corrupted:

{
    ...
    "manifest.json": "e82e8d958daa0d967c68897e00bf78bd884d1b1a",
    "signature": "ae8f34675a2f5a45a763e221bc8f9c54dc505548"
}

This PR makes a deep copy of the template files and lets the GC deal with removing the copy once the signing/compressing of the files is completed and the copy is not referenced anymore.

[alvinbaena]

It looks good to me. Thank you for finding the bug and making a PR.