Nhse o34 orkv.i70 case insensitive

[martinsumner]

Handles response headers in a case insensitive way - and returns as close as possible to same riakc_obj as returned in PB API.

Tested via the metadata_version_change test - OpenRiak/riak_test#14

[hmmr]

Looks good and fit for purpose to me. Before extracting bits from headers, we convert them into a map and convert them to lower case. A couple of points:

• there is one instance of an obsolescent string:tokens/2 left. Replace it with string:lexemes/2 while we are at it?
• move -include_lib("stdlib/include/assert.hrl"). within the -ifdef(TEST). block.

Also (and it's a reminder to myself, too), it will help reviewers if substantive changes are separated from the sweeping changes performed by erlfmt.