Address s3-test functional test suite failures

This pull request addresses the failures from #577. It requires related changes to `webmachine`, `mochiweb`, and the Basho fork of `s3-tests`.

Related PRs:

https://github.com/basho/mochiweb/pull/10
https://github.com/basho/webmachine/pull/150
https://github.com/basho/webmachine/pull/144
https://github.com/basho/s3-tests/pull/1

I have created a `rebar.config` that could be used to test these in concert if using a `rebar` built with the changes outlined in [this](https://github.com/rebar/rebar/pull/92) PR. 

```
{sub_dirs, ["rel"]}.

{require_otp_vsn, "R14B0[34]|R15"}.

{cover_enabled, true}.

{lib_dirs, ["deps", "apps"]}.

{erl_opts, [debug_info, warnings_as_errors, {parse_transform, lager_transform}]}.

{reset_after_eunit, true}.

{plugin_dir, ".plugins"}.
{plugins, [rebar_test_plugin]}.
{client_test, [
    {test_paths, ["client_tests/erlang"]},
    {test_output, ".client_test"}
]}.
{int_test, [
    {test_paths, ["int_test"]},
    {test_output, ".int_test"}
]}.
{riak_test, [
    {test_paths, ["riak_test/tests", "riak_test/src"]},
    {test_output, "riak_test/ebin"}
]}.

{deps, [
        {node_package, "1.2.2", {git, "git://github.com/basho/node_package", {tag, "1.2.2"}}},
	{mochiweb, ".*", {git, "git://github.com/basho/mochiweb", {branch, "klm-strip-header-values-take2"}}, [force_version]},
	{webmachine, ".*", {git, "git://github.com/basho/webmachine", {branch, "klm-handle-empty-content-type-rebase-testing-only"}}, [force_version]},
        {riakc, ".*", {git, "git://github.com/basho/riak-erlang-client", {tag, "1.3.1.1"}}},
        {lager, ".*", {git, "git://github.com/basho/lager", {tag, "1.2.2"}}},
        {eper, ".*", {git, "git://github.com/basho/eper.git", "3280b736057a6cf5c01590c79fd192f7e8645270"}},
        {sext, ".*", {git, "git://github.com/esl/sext", {tag, "0.4.1"}}},
        {druuid, ".*", {git, "git://github.com/kellymclaughlin/druuid.git", {tag, "0.2"}}},
        {velvet, "1.3.*", {git, "git://github.com/basho/velvet", {tag, "1.3.0"}}},
        {poolboy, "0.8.1", {git, "git://github.com/basho/poolboy", "0e15b5dcbff89b3d1d4021591f90375d4d2ee9a1"}},
        {folsom, ".*", {git, "git://github.com/boundary/folsom", {tag, "0.7.4"}}},
        {erlcloud, ".*", {git, "git://github.com/basho/erlcloud.git", {tag, "0.4.1p1"}}}
       ]}.
```

The `klm-handle-empty-content-type-rebase-testing-only` branch of webmachine is a branch I created just for this testing that combines the change sets from the two webmachine PRs mentioned above.