Missing conditional headers for some Blob service operations

[guangyang]

According to Specifying Conditional Headers for Blob Service Operations, the blob's acquire_lease method should support the conditional headers.

I assembled the following table from that Web page for auditing the PHP and Java blobs; it might be helpful for auditing the Ruby methods too:

REST Operation	If-Modified-Since	If-Unmodified-Since	If-Match	If-None-Match
Create Container	N/A	N/A	N/A	N/A
Delete Container	x	x	N/A	N/A
Get Container Metadata	N/A	N/A	N/A	N/A
Set Container Metadata	x	N/A	N/A	N/A
Get Container Properties	N/A	N/A	N/A	N/A
Get Container ACL	N/A	N/A	N/A	N/A
Set Container ACL	N/A	N/A	N/A	N/A
List Blobs	N/A	N/A	N/A	N/A
Get Blob Metadata	x	x	x	x
Set Blob Metadata	x	x	x	x
Get Blob Properties	x	x	N/A	N/A
Set Blob Properties	x	x	x	x
Put Blob	x	x	x	x
Get Blob	x	x	x	x
Copy Blob	x	x	x	x
Snapshot Blob	x	x	x	x
Lease Blob	x	x	x	x
Delete Blob	x	x	x	x
Put Block	N/A	N/A	N/A	N/A
Get Block List	x	x	x	x
Put Block List	x	x	x	x
Put Page	x	x	x	x
Get Page Ranges	x	x	x	x

For example, I just used it to find that the delete_blob is missing :if_modified_since and :if_unmodified_since.

[guangyang]

P3: Nice to fix