HEAD /Object?acl results in 500 error

```
commit e0aabc53bb7911526c84ae80770ba6f3cff44061 (HEAD, origin/release/1.3, release/1.3)
Merge: 9e8aaa8 48957cb
Author: Kazuhiro Suzuki <ksauzzmsg@gmail.com>
Date:   Sun Apr 7 19:34:10 2013 -0700

    Merge pull request #490 from basho/bugfix/kaz-update-ruby-test
```

After put an object, HEAD acl to the object:

```
$ s3curl.pl --head --id local_rcs -- -s -x 127.0.0.1:8080 'http://test.s3.amazonaws.com/1?acl'
HTTP/1.1 500 Internal Server Error
Server: Riak CS
Date: Tue, 09 Apr 2013 08:43:54 GMT
Content-Type: text/html
Content-Length: 170

```

Error log in Riak CS console:

```
16:32:34.613 [error] webmachine error: path="/buckets/test/objects/1/acl"
{error,{error,function_clause,[{riak_cs_wm_object_acl,produce_body,[{wm_reqdata,'HEAD',http,{1,1},"127.0.0.1",{wm_reqstate,#Port<0.7713>,{dict,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[[resource_module|riak_cs_wm_object_acl],['content-type',98,105,110,97,114,121,47,111,99,116,101,116,45,115,116,114,101,97,109]],[],[['content-encoding',105,100,101,110,116,105,116,121]],[],[],[],[],[],[],[],[]}}},undefined,"127.0.0.1",{wm_reqdata,'HEAD',http,{1,1},"127.0.0.1",{wm_reqstate,#Port<0.7713>,{dict,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[[resource_module|riak_cs_wm_object_acl],['content-type',98,105,110,97,114,121,47,111,99,116,101,116,45,115,116,114,101,97,109]],[],[['content-encoding',105,100,101,110,116,105,116,121]],[],[],[],[],[],[],[],[]}}},undefined,"127.0.0.1",{wm_reqdata,'HEAD',http,{1,1},"127.0.0.1",{wm_reqstate,#Port<0.7713>,{dict,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[[resource_module|riak_cs_wm_object_acl],['content-type',98,105,110,97,114,121,47,111,99,116,101,116,45,115,116,114,101,97,109]],[],[['content-encoding',105,100,101,110,116,105,116,121]],[],[],[],[],[],[],[],[]}}},undefined,"127.0.0.1",{wm_reqdata,'HEAD',http,{1,1},"127.0.0.1",{wm_reqstate,#Port<0.7713>,{dict,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[[resource_module|riak_cs_wm_object_acl],['content-type',98,105,110,97,114,121,47,111,99,116,101,116,45,115,116,114,101,97,109]],[],[['content-encoding',105,100,101,110,116,105,116,121]],[],[],[],[],[],[],[],[]}}},undefined,"127.0.0.1",{wm_reqdata,'HEAD',http,{1,1},"127.0.0.1",{wm_reqstate,#Port<0.7713>,{dict,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[[resource_module|riak_cs_wm_object_acl],['content-type',98,105,110,97,114,121,47,111,99,116,101,116,45,115,116,114,101,97,109]],[],[['content-encoding',105,100,101,110,116,105,116,121]],[],[],[],[],...}}},...},...},...},...},...},...},...},...},...},...},...],...},...]}}
[{riak_cs_wm_object_acl,produce_body,[{wm_reqdata,'HEAD',http,{1,1},"127.0.0.1",{wm_reqstate,#Port<0.7713>,{dict,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[[resource_module|riak_cs_wm_object_acl],['content-type',98,105,110,97,114,121,47,111,99,116,101,116,45,115,116,114,101,97,109]],[],[['content-encoding',105,100,101,110,116,105,116,121]],[],[],[],[],[],[],[],[]}}},undefined,"127.0.0.1",{wm_reqdata,'HEAD',http,{1,1},"127.0.0.1",{wm_reqstate,#Port<0.7713>,{dict,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[[resource_module|riak_cs_wm_object_acl],['content-type',98,105,110,97,114,121,47,111,99,116,101,116,45,115,116,114,101,97,109]],[],[['content-encoding',105,100,101,110,116,105,116,121]],[],[],[],[],[],[],[],[]}}},undefined,"127.0.0.1",{wm_reqdata,'HEAD',http,{1,1},"127.0.0.1",{wm_reqstate,#Port<0.7713>,{dict,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[[resource_module|riak_cs_wm_object_acl],['content-type',98,105,110,97,114,121,47,111,99,116,101,116,45,115,116,114,101,97,109]],[],[['content-encoding',105,100,101,110,116,105,116,121]],[],[],[],[],[],[],[],[]}}},undefined,"127.0.0.1",{wm_reqdata,'HEAD',http,{1,1},"127.0.0.1",{wm_reqstate,#Port<0.7713>,{dict,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[[resource_module|riak_cs_wm_object_acl],['content-type',98,105,110,97,114,121,47,111,99,116,101,116,45,115,116,114,101,97,109]],[],[['content-encoding',105,100,101,110,116,105,116,121]],[],[],[],[],[],[],[],[]}}},undefined,"127.0.0.1",{wm_reqdata,'HEAD',http,{1,1},"127.0.0.1",{wm_reqstate,#Port<0.7713>,{dict,3,16,16,8,80,48,{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},{{[],[],[],[],[],[[resource_module|riak_cs_wm_object_acl],['content-type',98,105,110,97,114,121,47,111,99,116,101,116,45,115,116,114,101,97,109]],[],[['content-encoding',105,100,...]],...}}},...},...},...},...}...
```