[storage] Adds support for Append Blobs

[BrianBland]

Adds PutAppendBlob and AppendBlock methods

Addresses #275

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[ahmetb]

@BrianBland thanks for the pull request. LGTM. extraHeaders will let users make use of x-ms-blob-condition-maxsize and x-ms-blob-condition-appendpos, which might be frequently used in the use cases of AppendBlob.

It looks like there's something funny going on about our storage tests. @colemickens have you changed something lately?

[BrianBland]

@ahmetalpbalkan it looks like I need to change some things up to support the new signing code as of version 2015-02-21 (see https://msdn.microsoft.com/en-us/library/azure/mt584141.aspx)

[ahmetb]

@BrianBland oh that explains the problem. We were actually planning to switch on SharedKeyLite signing (which is way simpler, doesn't have Content-Length either). If you don't mind making the change to work with 2015-02-21, please go ahead. We can do SharedKeyLite in a separate PR.

[BrianBland]

Hopefully you don't mind the repeated push trial and error with the CI system. I'm temporarily locked out of my Azure account 😞

[BrianBland]

@ahmetalpbalkan if you have any idea why TestBlobSASURICorrectness is failing with the new API version, I'm ready to fix it. I can't find any documentation suggesting why this is suddenly invalid.

[ahmetb]

@BrianBland oh that's just comparing a precomputed signature with the algorithm's output. You just need to change the hard-coded computed value with the result of then modified algorithm.

[BrianBland]

@ahmetalpbalkan I think you're referring to TestGetBlobSASURI. The failing test is getting a 403 status code from the server

[BrianBland]

Apparently the canonicalizedResource for the presigned blob urls must begin with "/blob", even though the documentation suggests that it should begin with "blob"

[ahmetb]

@BrianBland I'll follow up on that. Might be a docs issue indeed.

[ahmetb]

@BrianBland Hmm, I don't think you will still need this header now that you updated buildCanonicalizedString with

+   contentLength := headers["Content-Length"]
+   if contentLength == "0" {
+       contentLength = ""
+   }

In fact, there should be more occurrences of headers["Content-Length"] = "0" in this file, I think we can remove all those if you don't mind doing this in PR as well?

[BrianBland]

No worries, removed all of those

[ahmetb]

@BrianBland thanks LGTM!

[ahmetb]

@BrianBland hi there, just circling back on the docs issue https://msdn.microsoft.com/en-us/library/azure/dn140255.aspx I see that it says canonicalizedresource = "/blob/..., did you see a different value back then?

[BrianBland]

I thought so, but at least it looks correct now