Fix some deprecation warnings

There's a couple more that seem to be coming from the wrapped calls, but I'm puzzled why because  the `@c` macro doesn't explicitly use `convert`.

```
WARNING: convert{T}(p::Type{Ptr{T}},a::Array) is deprecated, use convert(p,pointer(a)) instead.
 in convert at deprecated.jl:26
 in exec_as_multi at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:670
 in _put_post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:484
 in put_post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:454
 in post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:388
 in post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:527
 in include_from_node1 at loading.jl:120
WARNING: convert{T}(p::Type{Ptr{T}},a::Array) is deprecated, use convert(p,pointer(a)) instead.
 in convert at deprecated.jl:26
 in exec_as_multi at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:673
 in _put_post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:484
 in put_post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:454
 in post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:388
 in post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:527
 in include_from_node1 at loading.jl:120
WARNING: convert{T}(p::Type{Ptr{T}},a::Array) is deprecated, use convert(p,pointer(a)) instead.
 in convert at deprecated.jl:26
 in process_response at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:202
 in exec_as_multi at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:704
 in _put_post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:484
 in put_post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:454
 in post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:388
 in post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:527
 in include_from_node1 at loading.jl:120
WARNING: convert{T}(p::Type{Ptr{T}},a::Array) is deprecated, use convert(p,pointer(a)) instead.
 in convert at deprecated.jl:26
 in process_response at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:202
 in exec_as_multi at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:704
 in _put_post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:484
 in put_post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:454
 in post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:388
 in post at /home/ihnorton/.julia/v0.3/HTTPClient/src/HTTPC.jl:527
 in include_from_node1 at loading.jl:120
```