Fix capabilities response for multiple client versions

[ishank011]

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[labkode]

@ishank011 is not so simple.

We need the following behaviour:

2.7+ will use Chunking V1 if "capabilities > files > bigfilechunking" is "true" AND "capabilities > dav > chunking" is not there
2.7+ will use Chunking NG if "capabilities > files > bigfilechunking" is "true" AND "capabilities > dav > chunking" = 1.0
2.7+ will use TUS if "capabilities > files > bigfilechunking" is "false" AND "capabilities > dav > chunking" = "" AND "capabilities > files > tus_support" has proper entries.

I propose something along these lines https://play.golang.org/p/V7N_v6qOv0e

[labkode]

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