Updating README.md to show Run Test as a section
Fixing mismatch in set_blob_properties options key

Fixing a discrepancy between documentation and implementation in
`blob_service.rb`

For the `set_blob_properties` method, the documentation specified
option keys to be of the form `:cache_control` while the code accessed
keys in the form `:blob_cache_control`.

It was unclear which form was correct, so I updated the documentation
to match the code
set_blob_properties validates options values

BlobService.set_blob_properties now raises an error if the options has
has an unexpected value. This is too correlate with the updated
documentation.

blob_service_test.rb test “does not modify the request headers when
provided an unknown value” (line 1250) has been updated to expect this
specific RuntimeError